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Subject[PATCH 4/7] lguest: documentation pt IV: Launcher
From
Date
Documentation: The Launcher

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

---
Documentation/lguest/lguest.c | 611 +++++++++++++++++++++++++++++++++++++----
drivers/lguest/core.c | 24 +
drivers/lguest/io.c | 249 +++++++++++++++-
drivers/lguest/lg.h | 25 +
drivers/lguest/lguest_user.c | 163 ++++++++++
5 files changed, 996 insertions(+), 76 deletions(-)

===================================================================
--- a/Documentation/lguest/lguest.c
+++ b/Documentation/lguest/lguest.c
@@ -34,12 +34,20 @@
#include <termios.h>
#include <getopt.h>
#include <zlib.h>
+/*L:110 We can ignore the 28 include files we need for this program, but I do
+ * want to draw attention to the use of kernel-style types.
+ *
+ * As Linus said, "C is a Spartan language, and so should your naming be." I
+ * like these abbreviations and the header we need uses them, so we define them
+ * here.
+ */
typedef unsigned long long u64;
typedef uint32_t u32;
typedef uint16_t u16;
typedef uint8_t u8;
#include "../../include/linux/lguest_launcher.h"
#include "../../include/asm-i386/e820.h"
+/*:*/

#define PAGE_PRESENT 0x7 /* Present, RW, Execute */
#define NET_PEERNUM 1
@@ -48,31 +56,46 @@ typedef uint8_t u8;
#define SIOCBRADDIF 0x89a2 /* add interface to bridge */
#endif

+/*L:120 verbose is both a global flag and a macro. The C preprocessor allows
+ * this, and although I wouldn't recommend it, it works quite nicely here. */
static bool verbose;
#define verbose(args...) \
do { if (verbose) printf(args); } while(0)
+/*:*/
static int waker_fd;

+
+/*This is our list of devices. */
struct device_list
{
+ /* Summary information about the devices in our list: ready to pass to
+ * select() to ask which need servicing.*/
fd_set infds;
int max_infd;

+ /* A single linked list of devices. */
struct device *dev;
+ /* ... And an end pointer so we can easily append new devices */
struct device **lastdev;
};

+/* The device structure describes a single device. */
struct device
{
+ /* The linked-list pointer. */
struct device *next;
+ /* The descriptor for this device, as mapped into the Guest. */
struct lguest_device_desc *desc;
+ /* The memory page(s) of this device, if any. Also mapped in Guest. */
void *mem;

- /* Watch this fd if handle_input non-NULL. */
+ /* If handle_input is set, it wants to be called when this file
+ * descriptor is ready. */
int fd;
bool (*handle_input)(int fd, struct device *me);

- /* Watch DMA to this key if handle_input non-NULL. */
+ /* If handle_output is set, it wants to be called when the Guest sends
+ * DMA to this key. */
unsigned long watch_key;
u32 (*handle_output)(int fd, const struct iovec *iov,
unsigned int num, struct device *me);
@@ -81,6 +104,11 @@ struct device
void *priv;
};

+/*L:130
+ * Loading the Kernel.
+ *
+ * We start with couple of simple helper routines. open_or_die() avoids
+ * error-checking code cluttering the callers: */
static int open_or_die(const char *name, int flags)
{
int fd = open(name, flags);
@@ -89,26 +117,38 @@ static int open_or_die(const char *name,
return fd;
}

+/* map_zeroed_pages() takes a (page-aligned) address and a number of pages. */
static void *map_zeroed_pages(unsigned long addr, unsigned int num)
{
+ /* We cache the /dev/zero file-descriptor so we only open it once. */
static int fd = -1;

if (fd == -1)
fd = open_or_die("/dev/zero", O_RDONLY);

+ /* We use a private mapping (ie. if we write to the page, it will be
+ * copied), and obviously we insist that it be mapped where we ask. */
if (mmap((void *)addr, getpagesize() * num,
PROT_READ|PROT_WRITE|PROT_EXEC, MAP_FIXED|MAP_PRIVATE, fd, 0)
!= (void *)addr)
err(1, "Mmaping %u pages of /dev/zero @%p", num, (void *)addr);
+
+ /* Returning the address is just a courtesy: can simplify callers. */
return (void *)addr;
}

-/* Find magic string marking entry point, return entry point. */
+/* To find out where to start we look for the magic Guest string, which marks
+ * the code we see in lguest_asm.S. This is a hack which we are currently
+ * plotting to replace with the normal Linux entry point. */
static unsigned long entry_point(void *start, void *end,
unsigned long page_offset)
{
void *p;

+ /* The scan gives us the physical starting address. We want the
+ * virtual address in this case, and fortunately, we already figured
+ * out the physical-virtual difference and passed it here in
+ * "page_offset". */
for (p = start; p < end; p++)
if (memcmp(p, "GenuineLguest", strlen("GenuineLguest")) == 0)
return (long)p + strlen("GenuineLguest") + page_offset;
@@ -116,7 +156,17 @@ static unsigned long entry_point(void *s
err(1, "Is this image a genuine lguest?");
}

-/* Returns the entry point */
+/* This routine takes an open vmlinux image, which is in ELF, and maps it into
+ * the Guest memory. ELF = Embedded Linking Format, which is the format used
+ * by all modern binaries on Linux including the kernel.
+ *
+ * The ELF headers give *two* addresses: a physical address, and a virtual
+ * address. The Guest kernel expects to be placed in memory at the physical
+ * address, and the page tables set up so it will correspond to that virtual
+ * address. We return the difference between the virtual and physical
+ * addresses in the "page_offset" pointer.
+ *
+ * We return the starting address. */
static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr,
unsigned long *page_offset)
{
@@ -125,40 +175,61 @@ static unsigned long map_elf(int elf_fd,
unsigned int i;
unsigned long start = -1UL, end = 0;

- /* Sanity checks. */
+ /* Sanity checks on the main ELF header: an x86 executable with a
+ * reasonable number of correctly-sized program headers. */
if (ehdr->e_type != ET_EXEC
|| ehdr->e_machine != EM_386
|| ehdr->e_phentsize != sizeof(Elf32_Phdr)
|| ehdr->e_phnum < 1 || ehdr->e_phnum > 65536U/sizeof(Elf32_Phdr))
errx(1, "Malformed elf header");

+ /* An ELF executable contains an ELF header and a number of "program"
+ * headers which indicate which parts ("segments") of the program to
+ * load where. */
+
+ /* We read in all the program headers at once: */
if (lseek(elf_fd, ehdr->e_phoff, SEEK_SET) < 0)
err(1, "Seeking to program headers");
if (read(elf_fd, phdr, sizeof(phdr)) != sizeof(phdr))
err(1, "Reading program headers");

+ /* We don't know page_offset yet. */
*page_offset = 0;
- /* We map the loadable segments at virtual addresses corresponding
- * to their physical addresses (our virtual == guest physical). */
+
+ /* Try all the headers: there are usually only three. A read-only one,
+ * a read-write one, and a "note" section which isn't loadable. */
for (i = 0; i < ehdr->e_phnum; i++) {
+ /* If this isn't a loadable segment, we ignore it */
if (phdr[i].p_type != PT_LOAD)
continue;

verbose("Section %i: size %i addr %p\n",
i, phdr[i].p_memsz, (void *)phdr[i].p_paddr);

- /* We expect linear address space. */
+ /* We expect a simple linear address space: every segment must
+ * have the same difference between virtual (p_vaddr) and
+ * physical (p_paddr) address. */
if (!*page_offset)
*page_offset = phdr[i].p_vaddr - phdr[i].p_paddr;
else if (*page_offset != phdr[i].p_vaddr - phdr[i].p_paddr)
errx(1, "Page offset of section %i different", i);

+ /* We track the first and last address we mapped, so we can
+ * tell entry_point() where to scan. */
if (phdr[i].p_paddr < start)
start = phdr[i].p_paddr;
if (phdr[i].p_paddr + phdr[i].p_filesz > end)
end = phdr[i].p_paddr + phdr[i].p_filesz;

- /* We map everything private, writable. */
+ /* We map this section of the file at its physical address. We
+ * map it read & write even if the header says this segment is
+ * read-only. The kernel really wants to be writable: it
+ * patches its own instructions which would normally be
+ * read-only.
+ *
+ * MAP_PRIVATE means that the page won't be copied until a
+ * write is done to it. This allows us to share much of the
+ * kernel memory between Guests. */
addr = mmap((void *)phdr[i].p_paddr,
phdr[i].p_filesz,
PROT_READ|PROT_WRITE|PROT_EXEC,
@@ -172,7 +243,31 @@ static unsigned long map_elf(int elf_fd,
return entry_point((void *)start, (void *)end, *page_offset);
}

-/* This is amazingly reliable. */
+/*L:170 Prepare to be SHOCKED and AMAZED. And possibly a trifle nauseated.
+ *
+ * We know that CONFIG_PAGE_OFFSET sets what virtual address the kernel expects
+ * to be. We don't know what that option was, but we can figure it out
+ * approximately by looking at the addresses in the code. I chose the common
+ * case of reading a memory location into the %eax register:
+ *
+ * movl <some-address>, %eax
+ *
+ * This gets encoded as five bytes: "0xA1 <4-byte-address>". For example,
+ * "0xA1 0x18 0x60 0x47 0xC0" reads the address 0xC0476018 into %eax.
+ *
+ * In this example can guess that the kernel was compiled with
+ * CONFIG_PAGE_OFFSET set to 0xC0000000 (it's always a round number). If the
+ * kernel were larger than 16MB, we might see 0xC1 addresses show up, but our
+ * kernel isn't that bloated yet.
+ *
+ * Unfortunately, x86 has variable-length instructions, so finding this
+ * particular instruction properly involves writing a disassembler. Instead,
+ * we rely on statistics. We look for "0xA1" and tally the different bytes
+ * which occur 4 bytes later (the "0xC0" in our example above). When one of
+ * those bytes appears three times, we can be reasonably confident that it
+ * forms the start of CONFIG_PAGE_OFFSET.
+ *
+ * This is amazingly reliable. */
static unsigned long intuit_page_offset(unsigned char *img, unsigned long len)
{
unsigned int i, possibilities[256] = { 0 };
@@ -185,30 +280,52 @@ static unsigned long intuit_page_offset(
errx(1, "could not determine page offset");
}

+/*L:160 Unfortunately the entire ELF image isn't compressed: the segments
+ * which need loading are extracted and compressed raw. This denies us the
+ * information we need to make a fully-general loader. */
static unsigned long unpack_bzimage(int fd, unsigned long *page_offset)
{
gzFile f;
int ret, len = 0;
+ /* A bzImage always gets loaded at physical address 1M. This is
+ * actually configurable as CONFIG_PHYSICAL_START, but as the comment
+ * there says, "Don't change this unless you know what you are doing".
+ * Indeed. */
void *img = (void *)0x100000;

+ /* gzdopen takes our file descriptor (carefully placed at the start of
+ * the GZIP header we found) and returns a gzFile. */
f = gzdopen(fd, "rb");
+ /* We read it into memory in 64k chunks until we hit the end. */
while ((ret = gzread(f, img + len, 65536)) > 0)
len += ret;
if (ret < 0)
err(1, "reading image from bzImage");

verbose("Unpacked size %i addr %p\n", len, img);
+
+ /* Without the ELF header, we can't tell virtual-physical gap. This is
+ * CONFIG_PAGE_OFFSET, and people do actually change it. Fortunately,
+ * I have a clever way of figuring it out from the code itself. */
*page_offset = intuit_page_offset(img, len);

return entry_point(img, img + len, *page_offset);
}

+/*L:150 A bzImage, unlike an ELF file, is not meant to be loaded. You're
+ * supposed to jump into it and it will unpack itself. We can't do that
+ * because the Guest can't run the unpacking code, and adding features to
+ * lguest kills puppies, so we don't want to.
+ *
+ * The bzImage is formed by putting the decompressing code in front of the
+ * compressed kernel code. So we can simple scan through it looking for the
+ * first "gzip" header, and start decompressing from there. */
static unsigned long load_bzimage(int fd, unsigned long *page_offset)
{
unsigned char c;
int state = 0;

- /* Ugly brute force search for gzip header. */
+ /* GZIP header is 0x1F 0x8B <method> <flags>... <compressed-by>. */
while (read(fd, &c, 1) == 1) {
switch (state) {
case 0:
@@ -225,8 +342,10 @@ static unsigned long load_bzimage(int fd
state++;
break;
case 9:
+ /* Seek back to the start of the gzip header. */
lseek(fd, -10, SEEK_CUR);
- if (c != 0x03) /* Compressed under UNIX. */
+ /* One final check: "compressed under UNIX". */
+ if (c != 0x03)
state = -1;
else
return unpack_bzimage(fd, page_offset);
@@ -235,25 +354,43 @@ static unsigned long load_bzimage(int fd
errx(1, "Could not find kernel in bzImage");
}

+/*L:140 Loading the kernel is easy when it's a "vmlinux", but most kernels
+ * come wrapped up in the self-decompressing "bzImage" format. With some funky
+ * coding, we can load those, too. */
static unsigned long load_kernel(int fd, unsigned long *page_offset)
{
Elf32_Ehdr hdr;

+ /* Read in the first few bytes. */
if (read(fd, &hdr, sizeof(hdr)) != sizeof(hdr))
err(1, "Reading kernel");

+ /* If it's an ELF file, it starts with "\177ELF" */
if (memcmp(hdr.e_ident, ELFMAG, SELFMAG) == 0)
return map_elf(fd, &hdr, page_offset);

+ /* Otherwise we assume it's a bzImage, and try to unpack it */
return load_bzimage(fd, page_offset);
}

+/* This is a trivial little helper to align pages. Andi Kleen hated it because
+ * it calls getpagesize() twice: "it's dumb code."
+ *
+ * Kernel guys get really het up about optimization, even when it's not
+ * necessary. I leave this code as a reaction against that. */
static inline unsigned long page_align(unsigned long addr)
{
+ /* Add upwards and truncate downwards. */
return ((addr + getpagesize()-1) & ~(getpagesize()-1));
}

-/* initrd gets loaded at top of memory: return length. */
+/*L:180 An "initial ram disk" is a disk image loaded into memory along with
+ * the kernel which the kernel can use to boot from without needing any
+ * drivers. Most distributions now use this as standard: the initrd contains
+ * the code to load the appropriate driver modules for the current machine.
+ *
+ * Importantly, James Morris works for RedHat, and Fedora uses initrds for its
+ * kernels. He sent me this (and tells me when I break it). */
static unsigned long load_initrd(const char *name, unsigned long mem)
{
int ifd;
@@ -262,21 +399,35 @@ static unsigned long load_initrd(const c
void *iaddr;

ifd = open_or_die(name, O_RDONLY);
+ /* fstat() is needed to get the file size. */
if (fstat(ifd, &st) < 0)
err(1, "fstat() on initrd '%s'", name);

+ /* The length needs to be rounded up to a page size: mmap needs the
+ * address to be page aligned. */
len = page_align(st.st_size);
+ /* We map the initrd at the top of memory. */
iaddr = mmap((void *)mem - len, st.st_size,
PROT_READ|PROT_EXEC|PROT_WRITE,
MAP_FIXED|MAP_PRIVATE, ifd, 0);
if (iaddr != (void *)mem - len)
err(1, "Mmaping initrd '%s' returned %p not %p",
name, iaddr, (void *)mem - len);
+ /* Once a file is mapped, you can close the file descriptor. It's a
+ * little odd, but quite useful. */
close(ifd);
verbose("mapped initrd %s size=%lu @ %p\n", name, st.st_size, iaddr);
+
+ /* We return the initrd size. */
return len;
}

+/* Once we know how much memory we have, and the address the Guest kernel
+ * expects, we can construct simple linear page tables which will get the Guest
+ * far enough into the boot to create its own.
+ *
+ * We lay them out of the way, just below the initrd (which is why we need to
+ * know its size). */
static unsigned long setup_pagetables(unsigned long mem,
unsigned long initrd_size,
unsigned long page_offset)
@@ -285,23 +436,32 @@ static unsigned long setup_pagetables(un
unsigned int mapped_pages, i, linear_pages;
unsigned int ptes_per_page = getpagesize()/sizeof(u32);

- /* If we can map all of memory above page_offset, we do so. */
+ /* Ideally we map all physical memory starting at page_offset.
+ * However, if page_offset is 0xC0000000 we can only map 1G of physical
+ * (0xC0000000 + 1G overflows). */
if (mem <= -page_offset)
mapped_pages = mem/getpagesize();
else
mapped_pages = -page_offset/getpagesize();

- /* Each linear PTE page can map ptes_per_page pages. */
+ /* Each PTE page can map ptes_per_page pages: how many do we need? */
linear_pages = (mapped_pages + ptes_per_page-1)/ptes_per_page;

- /* We lay out top-level then linear mapping immediately below initrd */
+ /* We put the toplevel page directory page at the top of memory. */
pgdir = (void *)mem - initrd_size - getpagesize();
+
+ /* Now we use the next linear_pages pages as pte pages */
linear = (void *)pgdir - linear_pages*getpagesize();

+ /* Linear mapping is easy: put every page's address into the mapping in
+ * order. PAGE_PRESENT contains the flags Present, Writable and
+ * Executable. */
for (i = 0; i < mapped_pages; i++)
linear[i] = ((i * getpagesize()) | PAGE_PRESENT);

- /* Now set up pgd so that this memory is at page_offset */
+ /* The top level points to the linear page table pages above. The
+ * entry representing page_offset points to the first one, and they
+ * continue from there. */
for (i = 0; i < mapped_pages; i += ptes_per_page) {
pgdir[(i + page_offset/getpagesize())/ptes_per_page]
= (((u32)linear + i*sizeof(u32)) | PAGE_PRESENT);
@@ -310,9 +470,13 @@ static unsigned long setup_pagetables(un
verbose("Linear mapping of %u pages in %u pte pages at %p\n",
mapped_pages, linear_pages, linear);

+ /* We return the top level (guest-physical) address: the kernel needs
+ * to know where it is. */
return (unsigned long)pgdir;
}

+/* Simple routine to roll all the commandline arguments together with spaces
+ * between them. */
static void concat(char *dst, char *args[])
{
unsigned int i, len = 0;
@@ -326,6 +490,10 @@ static void concat(char *dst, char *args
dst[len] = '\0';
}

+/* This is where we actually tell the kernel to initialize the Guest. We saw
+ * the arguments it expects when we looked at initialize() in lguest_user.c:
+ * the top physical page to allow, the top level pagetable, the entry point and
+ * the page_offset constant for the Guest. */
static int tell_kernel(u32 pgdir, u32 start, u32 page_offset)
{
u32 args[] = { LHREQ_INITIALIZE,
@@ -336,8 +504,11 @@ static int tell_kernel(u32 pgdir, u32 st
fd = open_or_die("/dev/lguest", O_RDWR);
if (write(fd, args, sizeof(args)) < 0)
err(1, "Writing to /dev/lguest");
+
+ /* We return the /dev/lguest file descriptor to control this Guest */
return fd;
}
+/*:*/

static void set_fd(int fd, struct device_list *devices)
{
@@ -346,61 +517,108 @@ static void set_fd(int fd, struct device
devices->max_infd = fd;
}

-/* When input arrives, we tell the kernel to kick lguest out with -EAGAIN. */
+/*L:200
+ * The Waker.
+ *
+ * With a console and network devices, we can have lots of input which we need
+ * to process. We could try to tell the kernel what file descriptors to watch,
+ * but handing a file descriptor mask through to the kernel is fairly icky.
+ *
+ * Instead, we fork off a process which watches the file descriptors and writes
+ * the LHREQ_BREAK command to the /dev/lguest filedescriptor to tell the Host
+ * loop to stop running the Guest. This causes it to return from the
+ * /dev/lguest read with -EAGAIN, where it will write to /dev/lguest to reset
+ * the LHREQ_BREAK and wake us up again.
+ *
+ * This, of course, is merely a different *kind* of icky.
+ */
static void wake_parent(int pipefd, int lguest_fd, struct device_list *devices)
{
+ /* Add the pipe from the Launcher to the fdset in the device_list, so
+ * we watch it, too. */
set_fd(pipefd, devices);

for (;;) {
fd_set rfds = devices->infds;
u32 args[] = { LHREQ_BREAK, 1 };

+ /* Wait until input is ready from one of the devices. */
select(devices->max_infd+1, &rfds, NULL, NULL, NULL);
+ /* Is it a message from the Launcher? */
if (FD_ISSET(pipefd, &rfds)) {
int ignorefd;
+ /* If read() returns 0, it means the Launcher has
+ * exited. We silently follow. */
if (read(pipefd, &ignorefd, sizeof(ignorefd)) == 0)
exit(0);
+ /* Otherwise it's telling us there's a problem with one
+ * of the devices, and we should ignore that file
+ * descriptor from now on. */
FD_CLR(ignorefd, &devices->infds);
- } else
+ } else /* Send LHREQ_BREAK command. */
write(lguest_fd, args, sizeof(args));
}
}

+/* This routine just sets up a pipe to the waker process. */
static int setup_waker(int lguest_fd, struct device_list *device_list)
{
int pipefd[2], child;

+ /* We create a pipe to talk to the waker, and also so it knows when the
+ * Launcher dies (and closes pipe). */
pipe(pipefd);
child = fork();
if (child == -1)
err(1, "forking");

if (child == 0) {
+ /* Close the "writing" end of our copy of the pipe */
close(pipefd[1]);
wake_parent(pipefd[0], lguest_fd, device_list);
}
+ /* Close the reading end of our copy of the pipe. */
close(pipefd[0]);

+ /* Here is the fd used to talk to the waker. */
return pipefd[1];
}

+/*L:210
+ * Device Handling.
+ *
+ * When the Guest sends DMA to us, it sends us an array of addresses and sizes.
+ * We need to make sure it's not trying to reach into the Launcher itself, so
+ * we have a convenient routine which check it and exits with an error message
+ * if something funny is going on:
+ */
static void *_check_pointer(unsigned long addr, unsigned int size,
unsigned int line)
{
+ /* We have to separately check addr and addr+size, because size could
+ * be huge and addr + size might wrap around. */
if (addr >= LGUEST_GUEST_TOP || addr + size >= LGUEST_GUEST_TOP)
errx(1, "%s:%i: Invalid address %li", __FILE__, line, addr);
+ /* We return a pointer for the caller's convenience, now we know it's
+ * safe to use. */
return (void *)addr;
}
+/* A macro which transparently hands the line number to the real function. */
#define check_pointer(addr,size) _check_pointer(addr, size, __LINE__)

-/* Returns pointer to dma->used_len */
+/* The Guest has given us the address of a "struct lguest_dma". We check it's
+ * OK and convert it to an iovec (which is a simple array of ptr/size
+ * pairs). */
static u32 *dma2iov(unsigned long dma, struct iovec iov[], unsigned *num)
{
unsigned int i;
struct lguest_dma *udma;

+ /* First we make sure that the array memory itself is valid. */
udma = check_pointer(dma, sizeof(*udma));
+ /* Now we check each element */
for (i = 0; i < LGUEST_MAX_DMA_SECTIONS; i++) {
+ /* A zero length ends the array. */
if (!udma->len[i])
break;

@@ -408,9 +626,15 @@ static u32 *dma2iov(unsigned long dma, s
iov[i].iov_len = udma->len[i];
}
*num = i;
+
+ /* We return the pointer to where the caller should write the amount of
+ * the buffer used. */
return &udma->used_len;
}

+/* This routine gets a DMA buffer from the Guest for a given key, and converts
+ * it to an iovec array. It returns the interrupt the Guest wants when we're
+ * finished, and a pointer to the "used_len" field to fill in. */
static u32 *get_dma_buffer(int fd, void *key,
struct iovec iov[], unsigned int *num, u32 *irq)
{
@@ -418,16 +642,21 @@ static u32 *get_dma_buffer(int fd, void
unsigned long udma;
u32 *res;

+ /* Ask the kernel for a DMA buffer corresponding to this key. */
udma = write(fd, buf, sizeof(buf));
+ /* They haven't registered any, or they're all used? */
if (udma == (unsigned long)-1)
return NULL;

- /* Kernel stashes irq in ->used_len. */
+ /* Convert it into our iovec array */
res = dma2iov(udma, iov, num);
+ /* The kernel stashes irq in ->used_len to get it out to us. */
*irq = *res;
+ /* Return a pointer to ((struct lguest_dma *)udma)->used_len. */
return res;
}

+/* This is a convenient routine to send the Guest an interrupt. */
static void trigger_irq(int fd, u32 irq)
{
u32 buf[] = { LHREQ_IRQ, irq };
@@ -435,6 +664,10 @@ static void trigger_irq(int fd, u32 irq)
err(1, "Triggering irq %i", irq);
}

+/* This simply sets up an iovec array where we can put data to be discarded.
+ * This happens when the Guest doesn't want or can't handle the input: we have
+ * to get rid of it somewhere, and if we bury it in the ceiling space it will
+ * start to smell after a week. */
static void discard_iovec(struct iovec *iov, unsigned int *num)
{
static char discard_buf[1024];
@@ -443,19 +676,24 @@ static void discard_iovec(struct iovec *
iov->iov_len = sizeof(discard_buf);
}

+/* Here is the input terminal setting we save, and the routine to restore them
+ * on exit so the user can see what they type next. */
static struct termios orig_term;
static void restore_term(void)
{
tcsetattr(STDIN_FILENO, TCSANOW, &orig_term);
}

+/* We associate some data with the console for our exit hack. */
struct console_abort
{
+ /* How many times have they hit ^C? */
int count;
+ /* When did they start? */
struct timeval start;
};

-/* We DMA input to buffer bound at start of console page. */
+/* This is the routine which handles console input (ie. stdin). */
static bool handle_console_input(int fd, struct device *dev)
{
u32 irq = 0, *lenp;
@@ -464,24 +702,38 @@ static bool handle_console_input(int fd,
struct iovec iov[LGUEST_MAX_DMA_SECTIONS];
struct console_abort *abort = dev->priv;

+ /* First we get the console buffer from the Guest. The key is dev->mem
+ * which was set to 0 in setup_console(). */
lenp = get_dma_buffer(fd, dev->mem, iov, &num, &irq);
if (!lenp) {
+ /* If it's not ready for input, warn and set up to discard. */
warn("console: no dma buffer!");
discard_iovec(iov, &num);
}

+ /* This is why we convert to iovecs: the readv() call uses them, and so
+ * it reads straight into the Guest's buffer. */
len = readv(dev->fd, iov, num);
if (len <= 0) {
+ /* This implies that the console is closed, is /dev/null, or
+ * something went terribly wrong. We still go through the rest
+ * of the logic, though, especially the exit handling below. */
warnx("Failed to get console input, ignoring console.");
len = 0;
}

+ /* If we read the data into the Guest, fill in the length and send the
+ * interrupt. */
if (lenp) {
*lenp = len;
trigger_irq(fd, irq);
}

- /* Three ^C within one second? Exit. */
+ /* Three ^C within one second? Exit.
+ *
+ * This is such a hack, but works surprisingly well. Each ^C has to be
+ * in a buffer by itself, so they can't be too fast. But we check that
+ * we get three within about a second, so they can't be too slow. */
if (len == 1 && ((char *)iov[0].iov_base)[0] == 3) {
if (!abort->count++)
gettimeofday(&abort->start, NULL);
@@ -489,43 +741,60 @@ static bool handle_console_input(int fd,
struct timeval now;
gettimeofday(&now, NULL);
if (now.tv_sec <= abort->start.tv_sec+1) {
- /* Make sure waker is not blocked in BREAK */
u32 args[] = { LHREQ_BREAK, 0 };
+ /* Close the fd so Waker will know it has to
+ * exit. */
close(waker_fd);
+ /* Just in case waker is blocked in BREAK, send
+ * unbreak now. */
write(fd, args, sizeof(args));
exit(2);
}
abort->count = 0;
}
} else
+ /* Any other key resets the abort counter. */
abort->count = 0;

+ /* Now, if we didn't read anything, put the input terminal back and
+ * return failure (meaning, don't call us again). */
if (!len) {
restore_term();
return false;
}
+ /* Everything went OK! */
return true;
}

+/* Handling console output is much simpler than input. */
static u32 handle_console_output(int fd, const struct iovec *iov,
unsigned num, struct device*dev)
{
+ /* Whatever the Guest sends, write it to standard output. Return the
+ * number of bytes written. */
return writev(STDOUT_FILENO, iov, num);
}

+/* Guest->Host network output is also pretty easy. */
static u32 handle_tun_output(int fd, const struct iovec *iov,
unsigned num, struct device *dev)
{
- /* Now we've seen output, we should warn if we can't get buffers. */
+ /* We put a flag in the "priv" pointer of the network device, and set
+ * it as soon as we see output. We'll see why in handle_tun_input() */
*(bool *)dev->priv = true;
+ /* Whatever packet the Guest sent us, write it out to the tun
+ * device. */
return writev(dev->fd, iov, num);
}

+/* This matches the peer_key() in lguest_net.c. The key for any given slot
+ * is the address of the network device's page plus 4 * the slot number. */
static unsigned long peer_offset(unsigned int peernum)
{
return 4 * peernum;
}

+/* This is where we handle a packet coming in from the tun device */
static bool handle_tun_input(int fd, struct device *dev)
{
u32 irq = 0, *lenp;
@@ -533,17 +802,28 @@ static bool handle_tun_input(int fd, str
unsigned num;
struct iovec iov[LGUEST_MAX_DMA_SECTIONS];

+ /* First we get a buffer the Guest has bound to its key. */
lenp = get_dma_buffer(fd, dev->mem+peer_offset(NET_PEERNUM), iov, &num,
&irq);
if (!lenp) {
+ /* Now, it's expected that if we try to send a packet too
+ * early, the Guest won't be ready yet. This is why we set a
+ * flag when the Guest sends its first packet. If it's sent a
+ * packet we assume it should be ready to receive them.
+ *
+ * Actually, this is what the status bits in the descriptor are
+ * for: we should *use* them. FIXME! */
if (*(bool *)dev->priv)
warn("network: no dma buffer!");
discard_iovec(iov, &num);
}

+ /* Read the packet from the device directly into the Guest's buffer. */
len = readv(dev->fd, iov, num);
if (len <= 0)
err(1, "reading network");
+
+ /* Write the used_len, and trigger the interrupt for the Guest */
if (lenp) {
*lenp = len;
trigger_irq(fd, irq);
@@ -551,9 +831,13 @@ static bool handle_tun_input(int fd, str
verbose("tun input packet len %i [%02x %02x] (%s)\n", len,
((u8 *)iov[0].iov_base)[0], ((u8 *)iov[0].iov_base)[1],
lenp ? "sent" : "discarded");
+ /* All good. */
return true;
}

+/* The last device handling routine is block output: the Guest has sent a DMA
+ * to the block device. It will have placed the command it wants in the
+ * "struct lguest_block_page". */
static u32 handle_block_output(int fd, const struct iovec *iov,
unsigned num, struct device *dev)
{
@@ -563,36 +847,64 @@ static u32 handle_block_output(int fd, c
struct iovec reply[LGUEST_MAX_DMA_SECTIONS];
off64_t device_len, off = (off64_t)p->sector * 512;

+ /* First we extract the device length from the dev->priv pointer. */
device_len = *(off64_t *)dev->priv;

+ /* We first check that the read or write is within the length of the
+ * block file. */
if (off >= device_len)
err(1, "Bad offset %llu vs %llu", off, device_len);
+ /* Move to the right location in the block file. This shouldn't fail,
+ * but best to check. */
if (lseek64(dev->fd, off, SEEK_SET) != off)
err(1, "Bad seek to sector %i", p->sector);

verbose("Block: %s at offset %llu\n", p->type ? "WRITE" : "READ", off);

+ /* They were supposed to bind a reply buffer at key equal to the start
+ * of the block device memory. We need this to tell them when the
+ * request is finished. */
lenp = get_dma_buffer(fd, dev->mem, reply, &reply_num, &irq);
if (!lenp)
err(1, "Block request didn't give us a dma buffer");

if (p->type) {
+ /* A write request. The DMA they sent contained the data, so
+ * write it out. */
len = writev(dev->fd, iov, num);
+ /* Grr... Now we know how long the "struct lguest_dma" they
+ * sent was, we make sure they didn't try to write over the end
+ * of the block file (possibly extending it). */
if (off + len > device_len) {
+ /* Trim it back to the correct length */
ftruncate(dev->fd, device_len);
+ /* Die, bad Guest, die. */
errx(1, "Write past end %llu+%u", off, len);
}
+ /* The reply length is 0: we just send back an empty DMA to
+ * interrupt them and tell them the write is finished. */
*lenp = 0;
} else {
+ /* A read request. They sent an empty DMA to start the
+ * request, and we put the read contents into the reply
+ * buffer. */
len = readv(dev->fd, reply, reply_num);
*lenp = len;
}

+ /* The result is 1 (done), 2 if there was an error (short read or
+ * write). */
p->result = 1 + (p->bytes != len);
+ /* Now tell them we've used their reply buffer. */
trigger_irq(fd, irq);
+
+ /* We're supposed to return the number of bytes of the output buffer we
+ * used. But the block device uses the "result" field instead, so we
+ * don't bother. */
return 0;
}

+/* This is the generic routine we call when the Guest sends some DMA out. */
static void handle_output(int fd, unsigned long dma, unsigned long key,
struct device_list *devices)
{
@@ -601,30 +913,53 @@ static void handle_output(int fd, unsign
struct iovec iov[LGUEST_MAX_DMA_SECTIONS];
unsigned num = 0;

+ /* Convert the "struct lguest_dma" they're sending to a "struct
+ * iovec". */
lenp = dma2iov(dma, iov, &num);
+
+ /* Check each device: if they expect output to this key, tell them to
+ * handle it. */
for (i = devices->dev; i; i = i->next) {
if (i->handle_output && key == i->watch_key) {
+ /* We write the result straight into the used_len field
+ * for them. */
*lenp = i->handle_output(fd, iov, num, i);
return;
}
}
+
+ /* This can happen: the kernel sends any SEND_DMA which doesn't match
+ * another Guest to us. It could be that another Guest just left a
+ * network, for example. But it's unusual. */
warnx("Pending dma %p, key %p", (void *)dma, (void *)key);
}

+/* This is called when the waker wakes us up: check for incoming file
+ * descriptors. */
static void handle_input(int fd, struct device_list *devices)
{
+ /* select() wants a zeroed timeval to mean "don't wait". */
struct timeval poll = { .tv_sec = 0, .tv_usec = 0 };

for (;;) {
struct device *i;
fd_set fds = devices->infds;

+ /* If nothing is ready, we're done. */
if (select(devices->max_infd+1, &fds, NULL, NULL, &poll) == 0)
break;

+ /* Otherwise, call the device(s) which have readable
+ * file descriptors and a method of handling them. */
for (i = devices->dev; i; i = i->next) {
if (i->handle_input && FD_ISSET(i->fd, &fds)) {
+ /* If handle_input() returns false, it means we
+ * should no longer service it.
+ * handle_console_input() does this. */
if (!i->handle_input(fd, i)) {
+ /* Clear it from the set of input file
+ * descriptors kept at the head of the
+ * device list. */
FD_CLR(i->fd, &devices->infds);
/* Tell waker to ignore it too... */
write(waker_fd, &i->fd, sizeof(i->fd));
@@ -634,9 +969,22 @@ static void handle_input(int fd, struct
}
}

+/*L:190
+ * Device Setup
+ *
+ * All devices need a descriptor so the Guest knows it exists, and a "struct
+ * device" so the Launcher can keep track of it. We have common helper
+ * routines to allocate them.
+ *
+ * This routine allocates a new "struct lguest_device_desc". This descriptor
+ * ultimitely belongs in the devices array just above the Guest's normal
+ * memory, but we set up devices while parsing the command line options and we
+ * don't know how much memory the Guest has yet. So we allocate them here and
+ * move them later in map_device_descriptors(). */
static struct lguest_device_desc *new_dev_desc(u16 type, u16 features,
u16 num_pages)
{
+ /* This is the very top allowed guest-physical address */
static unsigned long top = LGUEST_GUEST_TOP;
struct lguest_device_desc *desc;

@@ -644,7 +992,11 @@ static struct lguest_device_desc *new_de
desc->type = type;
desc->num_pages = num_pages;
desc->features = features;
+ /* The Guest sets status bits to indicate its progress. */
desc->status = 0;
+
+ /* If they said the device needs memory, we allocate that now, down
+ * from the top. */
if (num_pages) {
top -= num_pages*getpagesize();
map_zeroed_pages(top, num_pages);
@@ -654,6 +1006,9 @@ static struct lguest_device_desc *new_de
return desc;
}

+/* This monster routine does all the creation and setup of a new device,
+ * including caling new_dev_desc() to allocate the descriptor and device
+ * memory. */
static struct device *new_device(struct device_list *devices,
u16 type, u16 num_pages, u16 features,
int fd,
@@ -666,12 +1021,18 @@ static struct device *new_device(struct
{
struct device *dev = malloc(sizeof(*dev));

- /* Append to device list. */
+ /* Append to device list. Prepending to a single-linked list is
+ * easier, but the user expects the devices to be arranged on the bus
+ * in command-line order. The first network device on the command line
+ * is eth0, the first block device /dev/lgba, etc. */
*devices->lastdev = dev;
dev->next = NULL;
devices->lastdev = &dev->next;

+ /* Now we populate the fields one at a time. */
dev->fd = fd;
+ /* If we have an input handler for this file descriptor, then we add it
+ * to the device_list's fdset and maxfd. */
if (handle_input)
set_fd(dev->fd, devices);
dev->desc = new_dev_desc(type, features, num_pages);
@@ -682,27 +1043,37 @@ static struct device *new_device(struct
return dev;
}

+/* Our first setup routine is the console. It's a fairly simple device, but
+ * UNIX tty handling makes it uglier than it could be. */
static void setup_console(struct device_list *devices)
{
struct device *dev;

+ /* If we can save the initial standard input settings... */
if (tcgetattr(STDIN_FILENO, &orig_term) == 0) {
struct termios term = orig_term;
+ /* Then we turn off echo, line buffering and ^C etc. We want a
+ * raw input stream to the Guest. */
term.c_lflag &= ~(ISIG|ICANON|ECHO);
tcsetattr(STDIN_FILENO, TCSANOW, &term);
+ /* If we exit gracefully, the original settings will be
+ * restored so the user can see what they're typing. */
atexit(restore_term);
}

- /* We don't currently require a page for the console. */
+ /* We don't currently require any memory for the console, so we ask for
+ * 0 pages. */
dev = new_device(devices, LGUEST_DEVICE_T_CONSOLE, 0, 0,
STDIN_FILENO, handle_console_input,
LGUEST_CONSOLE_DMA_KEY, handle_console_output);
+ /* We store the console state in dev->priv, and initialize it. */
dev->priv = malloc(sizeof(struct console_abort));
((struct console_abort *)dev->priv)->count = 0;
verbose("device %p: console\n",
(void *)(dev->desc->pfn * getpagesize()));
}

+/* Setting up a block file is also fairly straightforward. */
static void setup_block_file(const char *filename, struct device_list *devices)
{
int fd;
@@ -710,20 +1081,47 @@ static void setup_block_file(const char
off64_t *device_len;
struct lguest_block_page *p;

+ /* We open with O_LARGEFILE because otherwise we get stuck at 2G. We
+ * open with O_DIRECT because otherwise our benchmarks go much too
+ * fast. */
fd = open_or_die(filename, O_RDWR|O_LARGEFILE|O_DIRECT);
+
+ /* We want one page, and have no input handler (the block file never
+ * has anything interesting to say to us). Our timing will be quite
+ * random, so it should be a reasonable randomness source. */
dev = new_device(devices, LGUEST_DEVICE_T_BLOCK, 1,
LGUEST_DEVICE_F_RANDOMNESS,
fd, NULL, 0, handle_block_output);
+
+ /* We store the device size in the private area */
device_len = dev->priv = malloc(sizeof(*device_len));
+ /* This is the safe way of establishing the size of our device: it
+ * might be a normal file or an actual block device like /dev/hdb. */
*device_len = lseek64(fd, 0, SEEK_END);
+
+ /* The device memory is a "struct lguest_block_page". It's zeroed
+ * already, we just need to put in the device size. Block devices
+ * think in sectors (ie. 512 byte chunks), so we translate here. */
p = dev->mem;
-
p->num_sectors = *device_len/512;
verbose("device %p: block %i sectors\n",
(void *)(dev->desc->pfn * getpagesize()), p->num_sectors);
}

-/* We use fnctl locks to reserve network slots (autocleanup!) */
+/*
+ * Network Devices.
+ *
+ * Setting up network devices is quite a pain, because we have three types.
+ * First, we have the inter-Guest network. This is a file which is mapped into
+ * the address space of the Guests who are on the network. Because it is a
+ * shared mapping, the same page underlies all the devices, and they can send
+ * DMA to each other.
+ *
+ * Remember from our network driver, the Guest is told what slot in the page it
+ * is to use. We use exclusive fnctl locks to reserve a slot. If another
+ * Guest is using a slot, the lock will fail and we try another. Because fnctl
+ * locks are cleaned up automatically when we die, this cleverly means that our
+ * reservation on the slot will vanish if we crash. */
static unsigned int find_slot(int netfd, const char *filename)
{
struct flock fl;
@@ -731,26 +1129,33 @@ static unsigned int find_slot(int netfd,
fl.l_type = F_WRLCK;
fl.l_whence = SEEK_SET;
fl.l_len = 1;
+ /* Try a 1 byte lock in each possible position number */
for (fl.l_start = 0;
fl.l_start < getpagesize()/sizeof(struct lguest_net);
fl.l_start++) {
+ /* If we succeed, return the slot number. */
if (fcntl(netfd, F_SETLK, &fl) == 0)
return fl.l_start;
}
errx(1, "No free slots in network file %s", filename);
}

+/* This function sets up the network file */
static void setup_net_file(const char *filename,
struct device_list *devices)
{
int netfd;
struct device *dev;

+ /* We don't use open_or_die() here: for friendliness we create the file
+ * if it doesn't already exist. */
netfd = open(filename, O_RDWR, 0);
if (netfd < 0) {
if (errno == ENOENT) {
netfd = open(filename, O_RDWR|O_CREAT, 0600);
if (netfd >= 0) {
+ /* If we succeeded, initialize the file with a
+ * blank page. */
char page[getpagesize()];
memset(page, 0, sizeof(page));
write(netfd, page, sizeof(page));
@@ -760,11 +1165,15 @@ static void setup_net_file(const char *f
err(1, "cannot open net file '%s'", filename);
}

+ /* We need 1 page, and the features indicate the slot to use and that
+ * no checksum is needed. We never touch this device again; it's
+ * between the Guests on the network, so we don't register input or
+ * output handlers. */
dev = new_device(devices, LGUEST_DEVICE_T_NET, 1,
find_slot(netfd, filename)|LGUEST_NET_F_NOCSUM,
-1, NULL, 0, NULL);

- /* We overwrite the /dev/zero mapping with the actual file. */
+ /* Map the shared file. */
if (mmap(dev->mem, getpagesize(), PROT_READ|PROT_WRITE,
MAP_FIXED|MAP_SHARED, netfd, 0) != dev->mem)
err(1, "could not mmap '%s'", filename);
@@ -772,6 +1181,7 @@ static void setup_net_file(const char *f
(void *)(dev->desc->pfn * getpagesize()), filename,
dev->desc->features & ~LGUEST_NET_F_NOCSUM);
}
+/*:*/

static u32 str2ip(const char *ipaddr)
{
@@ -781,7 +1191,11 @@ static u32 str2ip(const char *ipaddr)
return (byte[0] << 24) | (byte[1] << 16) | (byte[2] << 8) | byte[3];
}

-/* adapted from libbridge */
+/* This code is "adapted" from libbridge: it attaches the Host end of the
+ * network device to the bridge device specified by the command line.
+ *
+ * This is yet another James Morris contribution (I'm an IP-level guy, so I
+ * dislike bridging), and I just try not to break it. */
static void add_to_bridge(int fd, const char *if_name, const char *br_name)
{
int ifidx;
@@ -800,12 +1214,16 @@ static void add_to_bridge(int fd, const
err(1, "can't add %s to bridge %s", if_name, br_name);
}

+/* This sets up the Host end of the network device with an IP address, brings
+ * it up so packets will flow, the copies the MAC address into the hwaddr
+ * pointer (in practice, the Host's slot in the network device's memory). */
static void configure_device(int fd, const char *devname, u32 ipaddr,
unsigned char hwaddr[6])
{
struct ifreq ifr;
struct sockaddr_in *sin = (struct sockaddr_in *)&ifr.ifr_addr;

+ /* Don't read these incantations. Just cut & paste them like I did! */
memset(&ifr, 0, sizeof(ifr));
strcpy(ifr.ifr_name, devname);
sin->sin_family = AF_INET;
@@ -816,12 +1234,19 @@ static void configure_device(int fd, con
if (ioctl(fd, SIOCSIFFLAGS, &ifr) != 0)
err(1, "Bringing interface %s up", devname);

+ /* SIOC stands for Socket I/O Control. G means Get (vs S for Set
+ * above). IF means Interface, and HWADDR is hardware address.
+ * Simple! */
if (ioctl(fd, SIOCGIFHWADDR, &ifr) != 0)
err(1, "getting hw address for %s", devname);
-
memcpy(hwaddr, ifr.ifr_hwaddr.sa_data, 6);
}

+/*L:195 The other kind of network is a Host<->Guest network. This can either
+ * use briding or routing, but the principle is the same: it uses the "tun"
+ * device to inject packets into the Host as if they came in from a normal
+ * network card. We just shunt packets between the Guest and the tun
+ * device. */
static void setup_tun_net(const char *arg, struct device_list *devices)
{
struct device *dev;
@@ -830,36 +1255,56 @@ static void setup_tun_net(const char *ar
u32 ip;
const char *br_name = NULL;

+ /* We open the /dev/net/tun device and tell it we want a tap device. A
+ * tap device is like a tun device, only somehow different. To tell
+ * the truth, I completely blundered my way through this code, but it
+ * works now! */
netfd = open_or_die("/dev/net/tun", O_RDWR);
memset(&ifr, 0, sizeof(ifr));
ifr.ifr_flags = IFF_TAP | IFF_NO_PI;
strcpy(ifr.ifr_name, "tap%d");
if (ioctl(netfd, TUNSETIFF, &ifr) != 0)
err(1, "configuring /dev/net/tun");
+ /* We don't need checksums calculated for packets coming in this
+ * device: trust us! */
ioctl(netfd, TUNSETNOCSUM, 1);

- /* You will be peer 1: we should create enough jitter to randomize */
+ /* We create the net device with 1 page, using the features field of
+ * the descriptor to tell the Guest it is in slot 1 (NET_PEERNUM), and
+ * that the device has fairly random timing. We do *not* specify
+ * LGUEST_NET_F_NOCSUM: these packets can reach the real world.
+ *
+ * We will put our MAC address is slot 0 for the Guest to see, so
+ * it will send packets to us using the key "peer_offset(0)": */
dev = new_device(devices, LGUEST_DEVICE_T_NET, 1,
NET_PEERNUM|LGUEST_DEVICE_F_RANDOMNESS, netfd,
handle_tun_input, peer_offset(0), handle_tun_output);
+
+ /* We keep a flag which says whether we've seen packets come out from
+ * this network device. */
dev->priv = malloc(sizeof(bool));
*(bool *)dev->priv = false;

+ /* We need a socket to perform the magic network ioctls to bring up the
+ * tap interface, connect to the bridge etc. Any socket will do! */
ipfd = socket(PF_INET, SOCK_DGRAM, IPPROTO_IP);
if (ipfd < 0)
err(1, "opening IP socket");

+ /* If the command line was --tunnet=bridge:<name> do bridging. */
if (!strncmp(BRIDGE_PFX, arg, strlen(BRIDGE_PFX))) {
ip = INADDR_ANY;
br_name = arg + strlen(BRIDGE_PFX);
add_to_bridge(ipfd, ifr.ifr_name, br_name);
- } else
+ } else /* It is an IP address to set up the device with */
ip = str2ip(arg);

- /* We are peer 0, ie. first slot. */
+ /* We are peer 0, ie. first slot, so we hand dev->mem to this routine
+ * to write the MAC address at the start of the device memory. */
configure_device(ipfd, ifr.ifr_name, ip, dev->mem);

- /* Set "promisc" bit: we want every single packet. */
+ /* Set "promisc" bit: we want every single packet if we're going to
+ * bridge to other machines (and otherwise it doesn't matter). */
*((u8 *)dev->mem) |= 0x1;

close(ipfd);
@@ -871,7 +1316,10 @@ static void setup_tun_net(const char *ar
verbose("attached to bridge: %s\n", br_name);
}

-/* Now we know how much memory we have, we copy in device descriptors */
+/* That's the end of device setup.
+ *
+ * After we've finished all the command line processing we know how much memory
+ * we have, so we know where to put the device descriptors: */
static void map_device_descriptors(struct device_list *devs, unsigned long mem)
{
struct device *i;
@@ -889,12 +1337,18 @@ static void map_device_descriptors(struc
: i->desc->type == LGUEST_DEVICE_T_CONSOLE ? "console"
: i->desc->type == LGUEST_DEVICE_T_BLOCK ? "block"
: "unknown");
+ /* Copy the device descriptor into the Guest memory and change
+ * the "struct device"'s pointer to refer to the new copy.
+ * This means that the device's input and output routines can
+ * see the status bits. */
descs[num] = *i->desc;
free(i->desc);
i->desc = &descs[num];
}
}

+/*L:220 Finally we reach the core of the Launcher, which runs the Guest,
+ * serves its input and output, and finally, lays it to rest. */
static void __attribute__((noreturn))
run_guest(int lguest_fd, struct device_list *device_list)
{
@@ -906,20 +1360,37 @@ run_guest(int lguest_fd, struct device_l
/* We read from the /dev/lguest device to run the Guest. */
readval = read(lguest_fd, arr, sizeof(arr));

+ /* The read can only really return sizeof(arr) (the Guest did a
+ * SEND_DMA to us), or an error. */
+
+ /* For a successful read, arr[0] is the address of the "struct
+ * lguest_dma", and arr[1] is the key the Guest sent to. */
if (readval == sizeof(arr)) {
handle_output(lguest_fd, arr[0], arr[1], device_list);
continue;
+ /* ENOENT means the Guest died. Reading tells us why. */
} else if (errno == ENOENT) {
char reason[1024] = { 0 };
read(lguest_fd, reason, sizeof(reason)-1);
errx(1, "%s", reason);
+ /* EAGAIN means the waker wanted us to look at some input.
+ * Anything else means a bug or incompatible change. */
} else if (errno != EAGAIN)
err(1, "Running guest failed");
+
+ /* Service input, then unset the BREAK which releases
+ * the Waker. */
handle_input(lguest_fd, device_list);
if (write(lguest_fd, args, sizeof(args)) < 0)
err(1, "Resetting break");
}
}
+/*
+ * This is the end of the Launcher.
+ *
+ * But wait! We've seen I/O from the Launcher, and we've seen I/O from the
+ * Drivers. If we were to see the Host kernel I/O code, our understanding
+ * would be complete... :*/

static struct option opts[] = {
{ "verbose", 0, NULL, 'v' },
@@ -937,19 +1408,46 @@ static void usage(void)
"<mem-in-mb> vmlinux [args...]");
}

+/*L:100 The Launcher code itself takes us out into userspace, that scary place
+ * where pointers run wild and free! Unfortunately, like most userspace
+ * programs, it's quite boring (which is why everyone like to hack on the
+ * kernel!). Perhaps if you make up an Lguest Drinking Game at this point, it
+ * will get you through this section. Or, maybe not.
+ *
+ * The Launcher binary sits up high, usually starting at address 0xB8000000.
+ * Everything below this is the "physical" memory for the Guest. For example,
+ * if the Guest were to write a "1" at physical address 0, we would see a "1"
+ * in the Launcher at "(int *)0". Guest physical == Launcher virtual.
+ *
+ * This can be tough to get your head around, but usually it just means that we
+ * don't need to do any conversion when the Guest gives us it's "physical"
+ * addresses.
+ */
int main(int argc, char *argv[])
{
+ /* Memory, top-level pagetable, code startpoint, PAGE_OFFSET and size
+ * of the (optional) initrd. */
unsigned long mem, pgdir, start, page_offset, initrd_size = 0;
+ /* A temporary and the /dev/lguest file descriptor. */
int c, lguest_fd;
+ /* The list of Guest devices, based on command line arguments. */
struct device_list device_list;
+ /* The boot information for the Guest: at guest-physical address 0. */
void *boot = (void *)0;
+ /* If they specify an initrd file to load. */
const char *initrd_name = NULL;

+ /* First we initialize the device list. Since console and network
+ * device receive input from a file descriptor, we keep an fdset
+ * (infds) and the maximum fd number (max_infd) with the head of the
+ * list. We also keep a pointer to the last device, for easy appending
+ * to the list. */
device_list.max_infd = -1;
device_list.dev = NULL;
device_list.lastdev = &device_list.dev;
FD_ZERO(&device_list.infds);

+ /* The options are fairly straight-forward */
while ((c = getopt_long(argc, argv, "v", opts, NULL)) != EOF) {
switch (c) {
case 'v':
@@ -972,46 +1470,69 @@ int main(int argc, char *argv[])
usage();
}
}
+ /* After the other arguments we expect memory and kernel image name,
+ * followed by command line arguments for the kernel. */
if (optind + 2 > argc)
usage();

- /* We need a console device */
+ /* We always have a console device */
setup_console(&device_list);

- /* First we map /dev/zero over all of guest-physical memory. */
+ /* First remaining argument is the number of megabytes of memory. */
mem = atoi(argv[optind]) * 1024 * 1024;
+
+ /* We start by mapping anonymous pages over all of guest-physical
+ * memory range. This fills it with 0, and ensures that the Guest
+ * won't be killed when it tries to access it. */
map_zeroed_pages(0, mem / getpagesize());

/* Now we load the kernel */
start = load_kernel(open_or_die(argv[optind+1], O_RDONLY),
&page_offset);

- /* Write the device descriptors into memory. */
+ /* Write the device descriptors into memory for the Guest to know what
+ * devices it has. The descriptors are mapped just above normal
+ * memory: we couldn't do this while parsing the command-line arguments
+ * in setup_net_file() etc because we didn't know how much memory the
+ * Guest was going to have. */
map_device_descriptors(&device_list, mem);

- /* Map the initrd image if requested */
+ /* Map the initrd image if requested (at top of physical memory) */
if (initrd_name) {
initrd_size = load_initrd(initrd_name, mem);
+ /* These are the location in the Linux boot header where the
+ * start and size of the initrd are expected to be found. */
*(unsigned long *)(boot+0x218) = mem - initrd_size;
*(unsigned long *)(boot+0x21c) = initrd_size;
+ /* The bootloader type 0xFF means "unknown"; that's OK. */
*(unsigned char *)(boot+0x210) = 0xFF;
}

- /* Set up the initial linar pagetables. */
+ /* Set up the initial linear pagetables, starting below the initrd. */
pgdir = setup_pagetables(mem, initrd_size, page_offset);

- /* E820 memory map: ours is a simple, single region. */
+ /* The Linux boot header contains an "E820" memory map: ours is a
+ * simple, single region. */
*(char*)(boot+E820NR) = 1;
*((struct e820entry *)(boot+E820MAP))
= ((struct e820entry) { 0, mem, E820_RAM });
- /* Command line pointer and command line (at 4096) */
+ /* The boot header contains a command line pointer: we put the command
+ * line after the boot header (at address 4096) */
*(void **)(boot + 0x228) = boot + 4096;
concat(boot + 4096, argv+optind+2);
- /* Paravirt type: 1 == lguest */
+
+ /* The guest type value of "1" tells the Guest it's under lguest. */
*(int *)(boot + 0x23c) = 1;

+ /* We tell the kernel to initialize the Guest: this returns the open
+ * /dev/lguest file descriptor. */
lguest_fd = tell_kernel(pgdir, start, page_offset);
+
+ /* We fork off a child process, which wakes the Launcher whenever one
+ * of the input file descriptors needs attention. Otherwise we would
+ * run the Guest until it tries to output something. */
waker_fd = setup_waker(lguest_fd, &device_list);

+ /* Finally, run the Guest. This doesn't return. */
run_guest(lguest_fd, &device_list);
}
===================================================================
--- a/drivers/lguest/core.c
+++ b/drivers/lguest/core.c
@@ -208,24 +208,39 @@ static int emulate_insn(struct lguest *l
return 1;
}

+/*L:305
+ * Dealing With Guest Memory.
+ *
+ * When the Guest gives us (what it thinks is) a physical address, we can use
+ * the normal copy_from_user() & copy_to_user() on that address: remember,
+ * Guest physical == Launcher virtual.
+ *
+ * But we can't trust the Guest: it might be trying to access the Launcher
+ * code. We have to check that the range is below the pfn_limit the Launcher
+ * gave us. We have to make sure that addr + len doesn't give us a false
+ * positive by overflowing, too. */
int lguest_address_ok(const struct lguest *lg,
unsigned long addr, unsigned long len)
{
return (addr+len) / PAGE_SIZE < lg->pfn_limit && (addr+len >= addr);
}

-/* Just like get_user, but don't let guest access lguest binary. */
+/* This is a convenient routine to get a 32-bit value from the Guest (a very
+ * common operation). Here we can see how useful the kill_lguest() routine we
+ * met in the Launcher can be: we return a random value (0) instead of needing
+ * to return an error. */
u32 lgread_u32(struct lguest *lg, unsigned long addr)
{
u32 val = 0;

- /* Don't let them access lguest binary */
+ /* Don't let them access lguest binary. */
if (!lguest_address_ok(lg, addr, sizeof(val))
|| get_user(val, (u32 __user *)addr) != 0)
kill_guest(lg, "bad read address %#lx", addr);
return val;
}

+/* Same thing for writing a value. */
void lgwrite_u32(struct lguest *lg, unsigned long addr, u32 val)
{
if (!lguest_address_ok(lg, addr, sizeof(val))
@@ -233,6 +248,9 @@ void lgwrite_u32(struct lguest *lg, unsi
kill_guest(lg, "bad write address %#lx", addr);
}

+/* This routine is more generic, and copies a range of Guest bytes into a
+ * buffer. If the copy_from_user() fails, we fill the buffer with zeroes, so
+ * the caller doesn't end up using uninitialized kernel memory. */
void lgread(struct lguest *lg, void *b, unsigned long addr, unsigned bytes)
{
if (!lguest_address_ok(lg, addr, bytes)
@@ -243,6 +261,7 @@ void lgread(struct lguest *lg, void *b,
}
}

+/* Similarly, our generic routine to copy into a range of Guest bytes. */
void lgwrite(struct lguest *lg, unsigned long addr, const void *b,
unsigned bytes)
{
@@ -250,6 +269,7 @@ void lgwrite(struct lguest *lg, unsigned
|| copy_to_user((void __user *)addr, b, bytes) != 0)
kill_guest(lg, "bad write address %#lx len %u", addr, bytes);
}
+/* (end of memory access helper routines) :*/

static void set_ts(void)
{
===================================================================
--- a/drivers/lguest/io.c
+++ b/drivers/lguest/io.c
@@ -27,8 +27,36 @@
#include <linux/uaccess.h>
#include "lg.h"

+/*L:300
+ * I/O
+ *
+ * Getting data in and out of the Guest is quite an art. There are numerous
+ * ways to do it, and they all suck differently. We try to keep things fairly
+ * close to "real" hardware so our Guest's drivers don't look like an alien
+ * visitation in the middle of the Linux code, and yet make sure that Guests
+ * can talk directly to other Guests, not just the Launcher.
+ *
+ * To do this, the Guest gives us a key when it binds or sends DMA buffers.
+ * The key corresponds to a "physical" address inside the Guest (ie. a virtual
+ * address inside the Launcher process). We don't, however, use this key
+ * directly.
+ *
+ * We want Guests which share memory to be able to DMA to each other: two
+ * Launchers can mmap memory the same file, then the Guests can communicate.
+ * Fortunately, the futex code provides us with a way to get a "union
+ * futex_key" corresponding to the memory lying at a virtual address: if the
+ * two processes share memory, the "union futex_key" for that memory will match
+ * even if the memory is mapped at different addresses in each. So we always
+ * convert the keys to "union futex_key"s to compare them.
+ *
+ * Before we dive into this though, we need to look at another set of helper
+ * routines used throughout the Host kernel code to access Guest memory.
+ :*/
static struct list_head dma_hash[61];

+/* An unfortunate side effect of the Linux double-linked list implementation is
+ * that there's no good way to statically initialize an array of linked
+ * lists. */
void lguest_io_init(void)
{
unsigned int i;
@@ -60,6 +88,19 @@ kill:
return 0;
}

+/*L:330 This is our hash function, using the wonderful Jenkins hash.
+ *
+ * The futex key is a union with three parts: an unsigned long word, a pointer,
+ * and an int "offset". We could use jhash_2words() which takes three u32s.
+ * (Ok, the hash functions are great: the naming sucks though).
+ *
+ * It's nice to be portable to 64-bit platforms, so we use the more generic
+ * jhash2(), which takes an array of u32, the number of u32s, and an initial
+ * u32 to roll in. This is uglier, but breaks down to almost the same code on
+ * 32-bit platforms like this one.
+ *
+ * We want a position in the array, so we modulo ARRAY_SIZE(dma_hash) (ie. 61).
+ */
static unsigned int hash(const union futex_key *key)
{
return jhash2((u32*)&key->both.word,
@@ -68,6 +109,9 @@ static unsigned int hash(const union fut
% ARRAY_SIZE(dma_hash);
}

+/* This is a convenience routine to compare two keys. It's a much bemoaned C
+ * weakness that it doesn't allow '==' on structures or unions, so we have to
+ * open-code it like this. */
static inline int key_eq(const union futex_key *a, const union futex_key *b)
{
return (a->both.word == b->both.word
@@ -75,22 +119,36 @@ static inline int key_eq(const union fut
&& a->both.offset == b->both.offset);
}

-/* Must hold read lock on dmainfo owner's current->mm->mmap_sem */
+/*L:360 OK, when we need to actually free up a Guest's DMA array we do several
+ * things, so we have a convenient function to do it.
+ *
+ * The caller must hold a read lock on dmainfo owner's current->mm->mmap_sem
+ * for the drop_futex_key_refs(). */
static void unlink_dma(struct lguest_dma_info *dmainfo)
{
+ /* You locked this too, right? */
BUG_ON(!mutex_is_locked(&lguest_lock));
+ /* This is how we know that the entry is free. */
dmainfo->interrupt = 0;
+ /* Remove it from the hash table. */
list_del(&dmainfo->list);
+ /* Drop the references we were holding (to the inode or mm). */
drop_futex_key_refs(&dmainfo->key);
}

+/*L:350 This is the routine which we call when the Guest asks to unregister a
+ * DMA array attached to a given key. Returns true if the array was found. */
static int unbind_dma(struct lguest *lg,
const union futex_key *key,
unsigned long dmas)
{
int i, ret = 0;

+ /* We don't bother with the hash table, just look through all this
+ * Guest's DMA arrays. */
for (i = 0; i < LGUEST_MAX_DMA; i++) {
+ /* In theory it could have more than one array on the same key,
+ * or one array on multiple keys, so we check both */
if (key_eq(key, &lg->dma[i].key) && dmas == lg->dma[i].dmas) {
unlink_dma(&lg->dma[i]);
ret = 1;
@@ -100,51 +158,91 @@ static int unbind_dma(struct lguest *lg,
return ret;
}

+/*L:340 BIND_DMA: this is the hypercall which sets up an array of "struct
+ * lguest_dma" for receiving I/O.
+ *
+ * The Guest wants to bind an array of "struct lguest_dma"s to a particular key
+ * to receive input. This only happens when the Guest is setting up a new
+ * device, so it doesn't have to be very fast.
+ *
+ * It returns 1 on a successful registration (it can fail if we hit the limit
+ * of registrations for this Guest).
+ */
int bind_dma(struct lguest *lg,
unsigned long ukey, unsigned long dmas, u16 numdmas, u8 interrupt)
{
unsigned int i;
int ret = 0;
union futex_key key;
+ /* Futex code needs the mmap_sem. */
struct rw_semaphore *fshared = &current->mm->mmap_sem;

+ /* Invalid interrupt? (We could kill the guest here). */
if (interrupt >= LGUEST_IRQS)
return 0;

+ /* We need to grab the Big Lguest Lock, because other Guests may be
+ * trying to look through this Guest's DMAs to send something while
+ * we're doing this. */
mutex_lock(&lguest_lock);
down_read(fshared);
if (get_futex_key((u32 __user *)ukey, fshared, &key) != 0) {
kill_guest(lg, "bad dma key %#lx", ukey);
goto unlock;
}
+
+ /* We want to keep this key valid once we drop mmap_sem, so we have to
+ * hold a reference. */
get_futex_key_refs(&key);

+ /* If the Guest specified an interrupt of 0, that means they want to
+ * unregister this array of "struct lguest_dma"s. */
if (interrupt == 0)
ret = unbind_dma(lg, &key, dmas);
else {
+ /* Look through this Guest's dma array for an unused entry. */
for (i = 0; i < LGUEST_MAX_DMA; i++) {
+ /* If the interrupt is non-zero, the entry is already
+ * used. */
if (lg->dma[i].interrupt)
continue;

+ /* OK, a free one! Fill on our details. */
lg->dma[i].dmas = dmas;
lg->dma[i].num_dmas = numdmas;
lg->dma[i].next_dma = 0;
lg->dma[i].key = key;
lg->dma[i].guestid = lg->guestid;
lg->dma[i].interrupt = interrupt;
+
+ /* Now we add it to the hash table: the position
+ * depends on the futex key that we got. */
list_add(&lg->dma[i].list, &dma_hash[hash(&key)]);
+ /* Success! */
ret = 1;
goto unlock;
}
}
+ /* If we didn't find a slot to put the key in, drop the reference
+ * again. */
drop_futex_key_refs(&key);
unlock:
+ /* Unlock and out. */
up_read(fshared);
mutex_unlock(&lguest_lock);
return ret;
}

-/* lgread from another guest */
+/*L:385 Note that our routines to access a different Guest's memory are called
+ * lgread_other() and lgwrite_other(): these names emphasize that they are only
+ * used when the Guest is *not* the current Guest.
+ *
+ * The interface for copying from another process's memory is called
+ * access_process_vm(), with a final argument of 0 for a read, and 1 for a
+ * write.
+ *
+ * We need lgread_other() to read the destination Guest's "struct lguest_dma"
+ * array. */
static int lgread_other(struct lguest *lg,
void *buf, u32 addr, unsigned bytes)
{
@@ -157,7 +255,8 @@ static int lgread_other(struct lguest *l
return 1;
}

-/* lgwrite to another guest */
+/* "lgwrite()" to another Guest: used to update the destination "used_len" once
+ * we've transferred data into the buffer. */
static int lgwrite_other(struct lguest *lg, u32 addr,
const void *buf, unsigned bytes)
{
@@ -170,6 +269,15 @@ static int lgwrite_other(struct lguest *
return 1;
}

+/*L:400 This is the generic engine which copies from a source "struct
+ * lguest_dma" from this Guest into another Guest's "struct lguest_dma". The
+ * destination Guest's pages have already been mapped, as contained in the
+ * pages array.
+ *
+ * If you're wondering if there's a nice "copy from one process to another"
+ * routine, so was I. But Linux isn't really set up to copy between two
+ * unrelated processes, so we have to write it ourselves.
+ */
static u32 copy_data(struct lguest *srclg,
const struct lguest_dma *src,
const struct lguest_dma *dst,
@@ -178,33 +286,59 @@ static u32 copy_data(struct lguest *srcl
unsigned int totlen, si, di, srcoff, dstoff;
void *maddr = NULL;

+ /* We return the total length transferred. */
totlen = 0;
+
+ /* We keep indexes into the source and destination "struct lguest_dma",
+ * and an offset within each region. */
si = di = 0;
srcoff = dstoff = 0;
+
+ /* We loop until the source or destination is exhausted. */
while (si < LGUEST_MAX_DMA_SECTIONS && src->len[si]
&& di < LGUEST_MAX_DMA_SECTIONS && dst->len[di]) {
+ /* We can only transfer the rest of the src buffer, or as much
+ * as will fit into the destination buffer. */
u32 len = min(src->len[si] - srcoff, dst->len[di] - dstoff);

+ /* For systems using "highmem" we need to use kmap() to access
+ * the page we want. We often use the same page over and over,
+ * so rather than kmap() it on every loop, we set the maddr
+ * pointer to NULL when we need to move to the next
+ * destination page. */
if (!maddr)
maddr = kmap(pages[di]);

- /* FIXME: This is not completely portable, since
- archs do different things for copy_to_user_page. */
+ /* Copy directly from (this Guest's) source address to the
+ * destination Guest's kmap()ed buffer. Note that maddr points
+ * to the start of the page: we need to add the offset of the
+ * destination address and offset within the buffer. */
+
+ /* FIXME: This is not completely portable. I looked at
+ * copy_to_user_page(), and some arch's seem to need special
+ * flushes. x86 is fine. */
if (copy_from_user(maddr + (dst->addr[di] + dstoff)%PAGE_SIZE,
(void __user *)src->addr[si], len) != 0) {
+ /* If a copy failed, it's the source's fault. */
kill_guest(srclg, "bad address in sending DMA");
totlen = 0;
break;
}

+ /* Increment the total and src & dst offsets */
totlen += len;
srcoff += len;
dstoff += len;
+
+ /* Presumably we reached the end of the src or dest buffers: */
if (srcoff == src->len[si]) {
+ /* Move to the next buffer at offset 0 */
si++;
srcoff = 0;
}
if (dstoff == dst->len[di]) {
+ /* We need to unmap that destination page and reset
+ * maddr ready for the next one. */
kunmap(pages[di]);
maddr = NULL;
di++;
@@ -212,13 +346,15 @@ static u32 copy_data(struct lguest *srcl
}
}

+ /* If we still had a page mapped at the end, unmap now. */
if (maddr)
kunmap(pages[di]);

return totlen;
}

-/* Src is us, ie. current. */
+/*L:390 This is how we transfer a "struct lguest_dma" from the source Guest
+ * (the current Guest which called SEND_DMA) to another Guest. */
static u32 do_dma(struct lguest *srclg, const struct lguest_dma *src,
struct lguest *dstlg, const struct lguest_dma *dst)
{
@@ -226,23 +362,31 @@ static u32 do_dma(struct lguest *srclg,
u32 ret;
struct page *pages[LGUEST_MAX_DMA_SECTIONS];

+ /* We check that both source and destination "struct lguest_dma"s are
+ * within the bounds of the source and destination Guests */
if (!check_dma_list(dstlg, dst) || !check_dma_list(srclg, src))
return 0;

- /* First get the destination pages */
+ /* We need to map the pages which correspond to each parts of
+ * destination buffer. */
for (i = 0; i < LGUEST_MAX_DMA_SECTIONS; i++) {
if (dst->len[i] == 0)
break;
+ /* get_user_pages() is a complicated function, especially since
+ * we only want a single page. But it works, and returns the
+ * number of pages. Note that we're holding the destination's
+ * mmap_sem, as get_user_pages() requires. */
if (get_user_pages(dstlg->tsk, dstlg->mm,
dst->addr[i], 1, 1, 1, pages+i, NULL)
!= 1) {
+ /* This means the destination gave us a bogus buffer */
kill_guest(dstlg, "Error mapping DMA pages");
ret = 0;
goto drop_pages;
}
}

- /* Now copy until we run out of src or dst. */
+ /* Now copy the data until we run out of src or dst. */
ret = copy_data(srclg, src, dst, pages);

drop_pages:
@@ -251,6 +395,11 @@ drop_pages:
return ret;
}

+/*L:380 Transferring data from one Guest to another is not as simple as I'd
+ * like. We've found the "struct lguest_dma_info" bound to the same address as
+ * the send, we need to copy into it.
+ *
+ * This function returns true if the destination array was empty. */
static int dma_transfer(struct lguest *srclg,
unsigned long udma,
struct lguest_dma_info *dst)
@@ -259,15 +408,23 @@ static int dma_transfer(struct lguest *s
struct lguest *dstlg;
u32 i, dma = 0;

+ /* From the "struct lguest_dma_info" we found in the hash, grab the
+ * Guest. */
dstlg = &lguests[dst->guestid];
- /* Get our dma list. */
+ /* Read in the source "struct lguest_dma" handed to SEND_DMA. */
lgread(srclg, &src_dma, udma, sizeof(src_dma));

- /* We can't deadlock against them dmaing to us, because this
- * is all under the lguest_lock. */
+ /* We need the destination's mmap_sem, and we already hold the source's
+ * mmap_sem for the futex key lookup. Normally this would suggest that
+ * we could deadlock if the destination Guest was trying to send to
+ * this source Guest at the same time, which is another reason that all
+ * I/O is done under the big lguest_lock. */
down_read(&dstlg->mm->mmap_sem);

+ /* Look through the destination DMA array for an available buffer. */
for (i = 0; i < dst->num_dmas; i++) {
+ /* We keep a "next_dma" pointer which often helps us avoid
+ * looking at lots of previously-filled entries. */
dma = (dst->next_dma + i) % dst->num_dmas;
if (!lgread_other(dstlg, &dst_dma,
dst->dmas + dma * sizeof(struct lguest_dma),
@@ -277,30 +434,46 @@ static int dma_transfer(struct lguest *s
if (!dst_dma.used_len)
break;
}
+
+ /* If we found a buffer, we do the actual data copy. */
if (i != dst->num_dmas) {
unsigned long used_lenp;
unsigned int ret;

ret = do_dma(srclg, &src_dma, dstlg, &dst_dma);
- /* Put used length in src. */
+ /* Put used length in the source "struct lguest_dma"'s used_len
+ * field. It's a little tricky to figure out where that is,
+ * though. */
lgwrite_u32(srclg,
udma+offsetof(struct lguest_dma, used_len), ret);
+ /* Tranferring 0 bytes is OK if the source buffer was empty. */
if (ret == 0 && src_dma.len[0] != 0)
goto fail;

- /* Make sure destination sees contents before length. */
+ /* The destination Guest might be running on a different CPU:
+ * we have to make sure that it will see the "used_len" field
+ * change to non-zero *after* it sees the data we copied into
+ * the buffer. Hence a write memory barrier. */
wmb();
+ /* Figuring out where the destination's used_len field for this
+ * "struct lguest_dma" in the array is also a little ugly. */
used_lenp = dst->dmas
+ dma * sizeof(struct lguest_dma)
+ offsetof(struct lguest_dma, used_len);
lgwrite_other(dstlg, used_lenp, &ret, sizeof(ret));
+ /* Move the cursor for next time. */
dst->next_dma++;
}
up_read(&dstlg->mm->mmap_sem);

- /* Do this last so dst doesn't simply sleep on lock. */
+ /* We trigger the destination interrupt, even if the destination was
+ * empty and we didn't transfer anything: this gives them a chance to
+ * wake up and refill. */
set_bit(dst->interrupt, dstlg->irqs_pending);
+ /* Wake up the destination process. */
wake_up_process(dstlg->tsk);
+ /* If we passed the last "struct lguest_dma", the receive had no
+ * buffers left. */
return i == dst->num_dmas;

fail:
@@ -308,6 +481,8 @@ fail:
return 0;
}

+/*L:370 This is the counter-side to the BIND_DMA hypercall; the SEND_DMA
+ * hypercall. We find out who's listening, and send to them. */
void send_dma(struct lguest *lg, unsigned long ukey, unsigned long udma)
{
union futex_key key;
@@ -317,31 +492,43 @@ again:
again:
mutex_lock(&lguest_lock);
down_read(fshared);
+ /* Get the futex key for the key the Guest gave us */
if (get_futex_key((u32 __user *)ukey, fshared, &key) != 0) {
kill_guest(lg, "bad sending DMA key");
goto unlock;
}
- /* Shared mapping? Look for other guests... */
+ /* Since the key must be a multiple of 4, the futex key uses the lower
+ * bit of the "offset" field (which would always be 0) to indicate a
+ * mapping which is shared with other processes (ie. Guests). */
if (key.shared.offset & 1) {
struct lguest_dma_info *i;
+ /* Look through the hash for other Guests. */
list_for_each_entry(i, &dma_hash[hash(&key)], list) {
+ /* Don't send to ourselves. */
if (i->guestid == lg->guestid)
continue;
if (!key_eq(&key, &i->key))
continue;

+ /* If dma_transfer() tells us the destination has no
+ * available buffers, we increment "empty". */
empty += dma_transfer(lg, udma, i);
break;
}
+ /* If the destination is empty, we release our locks and
+ * give the destination Guest a brief chance to restock. */
if (empty == 1) {
/* Give any recipients one chance to restock. */
up_read(&current->mm->mmap_sem);
mutex_unlock(&lguest_lock);
+ /* Next time, we won't try again. */
empty++;
goto again;
}
} else {
- /* Private mapping: tell our userspace. */
+ /* Private mapping: Guest is sending to its Launcher. We set
+ * the "dma_is_pending" flag so that the main loop will exit
+ * and the Launcher's read() from /dev/lguest will return. */
lg->dma_is_pending = 1;
lg->pending_dma = udma;
lg->pending_key = ukey;
@@ -350,6 +537,7 @@ unlock:
up_read(fshared);
mutex_unlock(&lguest_lock);
}
+/*:*/

void release_all_dma(struct lguest *lg)
{
@@ -365,7 +553,8 @@ void release_all_dma(struct lguest *lg)
up_read(&lg->mm->mmap_sem);
}

-/* Userspace wants a dma buffer from this guest. */
+/*L:320 This routine looks for a DMA buffer registered by the Guest on the
+ * given key (using the BIND_DMA hypercall). */
unsigned long get_dma_buffer(struct lguest *lg,
unsigned long ukey, unsigned long *interrupt)
{
@@ -374,15 +563,29 @@ unsigned long get_dma_buffer(struct lgue
struct lguest_dma_info *i;
struct rw_semaphore *fshared = &current->mm->mmap_sem;

+ /* Take the Big Lguest Lock to stop other Guests sending this Guest DMA
+ * at the same time. */
mutex_lock(&lguest_lock);
+ /* To match between Guests sharing the same underlying memory we steal
+ * code from the futex infrastructure. This requires that we hold the
+ * "mmap_sem" for our process (the Launcher), and pass it to the futex
+ * code. */
down_read(fshared);
+
+ /* This can fail if it's not a valid address, or if the address is not
+ * divisible by 4 (the futex code needs that, we don't really). */
if (get_futex_key((u32 __user *)ukey, fshared, &key) != 0) {
kill_guest(lg, "bad registered DMA buffer");
goto unlock;
}
+ /* Search the hash table for matching entries (the Launcher can only
+ * send to its own Guest for the moment, so the entry must be for this
+ * Guest) */
list_for_each_entry(i, &dma_hash[hash(&key)], list) {
if (key_eq(&key, &i->key) && i->guestid == lg->guestid) {
unsigned int j;
+ /* Look through the registered DMA array for an
+ * available buffer. */
for (j = 0; j < i->num_dmas; j++) {
struct lguest_dma dma;

@@ -391,6 +594,8 @@ unsigned long get_dma_buffer(struct lgue
if (dma.used_len == 0)
break;
}
+ /* Store the interrupt the Guest wants when the buffer
+ * is used. */
*interrupt = i->interrupt;
break;
}
@@ -400,4 +605,12 @@ unlock:
mutex_unlock(&lguest_lock);
return ret;
}
-
+/*:*/
+
+/*L:410 This really has completed the Launcher. Not only have we now finished
+ * the longest chapter in our journey, but this also means we are over halfway
+ * through!
+ *
+ * Enough prevaricating around the bush: it is time for us to dive into the
+ * core of the Host, in "make Host".
+ */
===================================================================
--- a/drivers/lguest/lg.h
+++ b/drivers/lguest/lg.h
@@ -244,6 +244,30 @@ unsigned long get_dma_buffer(struct lgue
/* hypercalls.c: */
void do_hypercalls(struct lguest *lg);

+/*L:035
+ * Let's step aside for the moment, to study one important routine that's used
+ * widely in the Host code.
+ *
+ * There are many cases where the Guest does something invalid, like pass crap
+ * to a hypercall. Since only the Guest kernel can make hypercalls, it's quite
+ * acceptable to simply terminate the Guest and give the Launcher a nicely
+ * formatted reason. It's also simpler for the Guest itself, which doesn't
+ * need to check most hypercalls for "success"; if you're still running, it
+ * succeeded.
+ *
+ * Once this is called, the Guest will never run again, so most Host code can
+ * call this then continue as if nothing had happened. This means many
+ * functions don't have to explicitly return an error code, which keeps the
+ * code simple.
+ *
+ * It also means that this can be called more than once: only the first one is
+ * remembered. The only trick is that we still need to kill the Guest even if
+ * we can't allocate memory to store the reason. Linux has a neat way of
+ * packing error codes into invalid pointers, so we use that here.
+ *
+ * Like any macro which uses an "if", it is safely wrapped in a run-once "do {
+ * } while(0)".
+ */
#define kill_guest(lg, fmt...) \
do { \
if (!(lg)->dead) { \
@@ -252,6 +276,7 @@ do { \
(lg)->dead = ERR_PTR(-ENOMEM); \
} \
} while(0)
+/* (End of aside) :*/

static inline unsigned long guest_pa(struct lguest *lg, unsigned long vaddr)
{
===================================================================
--- a/drivers/lguest/lguest_user.c
+++ b/drivers/lguest/lguest_user.c
@@ -9,33 +9,62 @@
#include <linux/fs.h>
#include "lg.h"

+/*L:030 setup_regs() doesn't really belong in this file, but it gives us an
+ * early glimpse deeper into the Host so it's worth having here.
+ *
+ * Most of the Guest's registers are left alone: we used get_zeroed_page() to
+ * allocate the structure, so they will be 0. */
static void setup_regs(struct lguest_regs *regs, unsigned long start)
{
- /* Write out stack in format lguest expects, so we can switch to it. */
+ /* There are four "segment" registers which the Guest needs to boot:
+ * The "code segment" register (cs) refers to the kernel code segment
+ * __KERNEL_CS, and the "data", "extra" and "stack" segment registers
+ * refer to the kernel data segment __KERNEL_DS.
+ *
+ * The privilege level is packed into the lower bits. The Guest runs
+ * at privilege level 1 (GUEST_PL).*/
regs->ds = regs->es = regs->ss = __KERNEL_DS|GUEST_PL;
regs->cs = __KERNEL_CS|GUEST_PL;
- regs->eflags = 0x202; /* Interrupts enabled. */
+
+ /* The "eflags" register contains miscellaneous flags. Bit 1 (0x002)
+ * is supposed to always be "1". Bit 9 (0x200) controls whether
+ * interrupts are enabled. We always leave interrupts enabled while
+ * running the Guest. */
+ regs->eflags = 0x202;
+
+ /* The "Extended Instruction Pointer" register says where the Guest is
+ * running. */
regs->eip = start;
- /* esi points to our boot information (physical address 0) */
-}
-
-/* + addr */
+
+ /* %esi points to our boot information, at physical address 0, so don't
+ * touch it. */
+}
+
+/*L:310 To send DMA into the Guest, the Launcher needs to be able to ask for a
+ * DMA buffer. This is done by writing LHREQ_GETDMA and the key to
+ * /dev/lguest. */
static long user_get_dma(struct lguest *lg, const u32 __user *input)
{
unsigned long key, udma, irq;

+ /* Fetch the key they wrote to us. */
if (get_user(key, input) != 0)
return -EFAULT;
+ /* Look for a free Guest DMA buffer bound to that key. */
udma = get_dma_buffer(lg, key, &irq);
if (!udma)
return -ENOENT;

- /* We put irq number in udma->used_len. */
+ /* We need to tell the Launcher what interrupt the Guest expects after
+ * the buffer is filled. We stash it in udma->used_len. */
lgwrite_u32(lg, udma + offsetof(struct lguest_dma, used_len), irq);
+
+ /* The (guest-physical) address of the DMA buffer is returned from
+ * the write(). */
return udma;
}

-/* To force the Guest to stop running and return to the Launcher, the
+/*L:315 To force the Guest to stop running and return to the Launcher, the
* Waker sets writes LHREQ_BREAK and the value "1" to /dev/lguest. The
* Launcher then writes LHREQ_BREAK and "0" to release the Waker. */
static int break_guest_out(struct lguest *lg, const u32 __user *input)
@@ -59,7 +88,8 @@ static int break_guest_out(struct lguest
}
}

-/* + irq */
+/*L:050 Sending an interrupt is done by writing LHREQ_IRQ and an interrupt
+ * number to /dev/lguest. */
static int user_send_irq(struct lguest *lg, const u32 __user *input)
{
u32 irq;
@@ -68,14 +98,19 @@ static int user_send_irq(struct lguest *
return -EFAULT;
if (irq >= LGUEST_IRQS)
return -EINVAL;
+ /* Next time the Guest runs, the core code will see if it can deliver
+ * this interrupt. */
set_bit(irq, lg->irqs_pending);
return 0;
}

+/*L:040 Once our Guest is initialized, the Launcher makes it run by reading
+ * from /dev/lguest. */
static ssize_t read(struct file *file, char __user *user, size_t size,loff_t*o)
{
struct lguest *lg = file->private_data;

+ /* You must write LHREQ_INITIALIZE first! */
if (!lg)
return -EINVAL;

@@ -83,27 +118,52 @@ static ssize_t read(struct file *file, c
if (current != lg->tsk)
return -EPERM;

+ /* If the guest is already dead, we indicate why */
if (lg->dead) {
size_t len;

+ /* lg->dead either contains an error code, or a string. */
if (IS_ERR(lg->dead))
return PTR_ERR(lg->dead);

+ /* We can only return as much as the buffer they read with. */
len = min(size, strlen(lg->dead)+1);
if (copy_to_user(user, lg->dead, len) != 0)
return -EFAULT;
return len;
}

+ /* If we returned from read() last time because the Guest sent DMA,
+ * clear the flag. */
if (lg->dma_is_pending)
lg->dma_is_pending = 0;

+ /* Run the Guest until something interesting happens. */
return run_guest(lg, (unsigned long __user *)user);
}

-/* Take: pfnlimit, pgdir, start, pageoffset. */
+/*L:020 The initialization write supplies 4 32-bit values (in addition to the
+ * 32-bit LHREQ_INITIALIZE value). These are:
+ *
+ * pfnlimit: The highest (Guest-physical) page number the Guest should be
+ * allowed to access. The Launcher has to live in Guest memory, so it sets
+ * this to ensure the Guest can't reach it.
+ *
+ * pgdir: The (Guest-physical) address of the top of the initial Guest
+ * pagetables (which are set up by the Launcher).
+ *
+ * start: The first instruction to execute ("eip" in x86-speak).
+ *
+ * page_offset: The PAGE_OFFSET constant in the Guest kernel. We should
+ * probably wean the code off this, but it's a very useful constant! Any
+ * address above this is within the Guest kernel, and any kernel address can
+ * quickly converted from physical to virtual by adding PAGE_OFFSET. It's
+ * 0xC0000000 (3G) by default, but it's configurable at kernel build time.
+ */
static int initialize(struct file *file, const u32 __user *input)
{
+ /* "struct lguest" contains everything we (the Host) know about a
+ * Guest. */
struct lguest *lg;
int err, i;
u32 args[4];
@@ -111,7 +171,7 @@ static int initialize(struct file *file,
/* We grab the Big Lguest lock, which protects the global array
* "lguests" and multiple simultaneous initializations. */
mutex_lock(&lguest_lock);
-
+ /* You can't initialize twice! Close the device and start again... */
if (file->private_data) {
err = -EBUSY;
goto unlock;
@@ -122,37 +182,70 @@ static int initialize(struct file *file,
goto unlock;
}

+ /* Find an unused guest. */
i = find_free_guest();
if (i < 0) {
err = -ENOSPC;
goto unlock;
}
+ /* OK, we have an index into the "lguest" array: "lg" is a convenient
+ * pointer. */
lg = &lguests[i];
+
+ /* Populate the easy fields of our "struct lguest" */
lg->guestid = i;
lg->pfn_limit = args[0];
lg->page_offset = args[3];
+
+ /* We need a complete page for the Guest registers: they are accessible
+ * to the Guest and we can only grant it access to whole pages. */
lg->regs_page = get_zeroed_page(GFP_KERNEL);
if (!lg->regs_page) {
err = -ENOMEM;
goto release_guest;
}
+ /* We actually put the registers at the bottom of the page. */
lg->regs = (void *)lg->regs_page + PAGE_SIZE - sizeof(*lg->regs);

+ /* Initialize the Guest's shadow page tables, using the toplevel
+ * address the Launcher gave us. This allocates memory, so can
+ * fail. */
err = init_guest_pagetable(lg, args[1]);
if (err)
goto free_regs;

+ /* Now we initialize the Guest's registers, handing it the start
+ * address. */
setup_regs(lg->regs, args[2]);
+
+ /* There are a couple of GDT entries the Guest expects when first
+ * booting. */
setup_guest_gdt(lg);
+
+ /* The timer for lguest's clock needs initialization. */
init_clockdev(lg);
+
+ /* We keep a pointer to the Launcher task (ie. current task) for when
+ * other Guests want to wake this one (inter-Guest I/O). */
lg->tsk = current;
+ /* We need to keep a pointer to the Launcher's memory map, because if
+ * the Launcher dies we need to clean it up. If we don't keep a
+ * reference, it is destroyed before close() is called. */
lg->mm = get_task_mm(lg->tsk);
+
+ /* Initialize the queue for the waker to wait on */
init_waitqueue_head(&lg->break_wq);
+
+ /* We remember which CPU's pages this Guest used last, for optimization
+ * when the same Guest runs on the same CPU twice. */
lg->last_pages = NULL;
+
+ /* We keep our "struct lguest" in the file's private_data. */
file->private_data = lg;

mutex_unlock(&lguest_lock);

+ /* And because this is a write() call, we return the length used. */
return sizeof(args);

free_regs:
@@ -164,9 +257,15 @@ unlock:
return err;
}

+/*L:010 The first operation the Launcher does must be a write. All writes
+ * start with a 32 bit number: for the first write this must be
+ * LHREQ_INITIALIZE to set up the Guest. After that the Launcher can use
+ * writes of other values to get DMA buffers and send interrupts. */
static ssize_t write(struct file *file, const char __user *input,
size_t size, loff_t *off)
{
+ /* Once the guest is initialized, we hold the "struct lguest" in the
+ * file private data. */
struct lguest *lg = file->private_data;
u32 req;

@@ -174,8 +273,11 @@ static ssize_t write(struct file *file,
return -EFAULT;
input += sizeof(req);

+ /* If you haven't initialized, you must do that first. */
if (req != LHREQ_INITIALIZE && !lg)
return -EINVAL;
+
+ /* Once the Guest is dead, all you can do is read() why it died. */
if (lg && lg->dead)
return -ENOENT;

@@ -197,33 +299,72 @@ static ssize_t write(struct file *file,
}
}

+/*L:060 The final piece of interface code is the close() routine. It reverses
+ * everything done in initialize(). This is usually called because the
+ * Launcher exited.
+ *
+ * Note that the close routine returns 0 or a negative error number: it can't
+ * really fail, but it can whine. I blame Sun for this wart, and K&R C for
+ * letting them do it. :*/
static int close(struct inode *inode, struct file *file)
{
struct lguest *lg = file->private_data;

+ /* If we never successfully initialized, there's nothing to clean up */
if (!lg)
return 0;

+ /* We need the big lock, to protect from inter-guest I/O and other
+ * Launchers initializing guests. */
mutex_lock(&lguest_lock);
/* Cancels the hrtimer set via LHCALL_SET_CLOCKEVENT. */
hrtimer_cancel(&lg->hrt);
+ /* Free any DMA buffers the Guest had bound. */
release_all_dma(lg);
+ /* Free up the shadow page tables for the Guest. */
free_guest_pagetable(lg);
+ /* Now all the memory cleanups are done, it's safe to release the
+ * Launcher's memory management structure. */
mmput(lg->mm);
+ /* If lg->dead doesn't contain an error code it will be NULL or a
+ * kmalloc()ed string, either of which is ok to hand to kfree(). */
if (!IS_ERR(lg->dead))
kfree(lg->dead);
+ /* We can free up the register page we allocated. */
free_page(lg->regs_page);
+ /* We clear the entire structure, which also marks it as free for the
+ * next user. */
memset(lg, 0, sizeof(*lg));
+ /* Release lock and exit. */
mutex_unlock(&lguest_lock);
+
return 0;
}

+/*L:000
+ * Welcome to our journey through the Launcher!
+ *
+ * The Launcher is the Host userspace program which sets up, runs and services
+ * the Guest. In fact, many comments in the Drivers which refer to "the Host"
+ * doing things are inaccurate: the Launcher does all the device handling for
+ * the Guest. The Guest can't tell what's done by the the Launcher and what by
+ * the Host.
+ *
+ * Just to confuse you: to the Host kernel, the Launcher *is* the Guest and we
+ * shall see more of that later.
+ *
+ * We begin our understanding with the Host kernel interface which the Launcher
+ * uses: reading and writing a character device called /dev/lguest. All the
+ * work happens in the read(), write() and close() routines: */
static struct file_operations lguest_fops = {
.owner = THIS_MODULE,
.release = close,
.write = write,
.read = read,
};
+
+/* This is a textbook example of a "misc" character device. Populate a "struct
+ * miscdevice" and register it with misc_register(). */
static struct miscdevice lguest_dev = {
.minor = MISC_DYNAMIC_MINOR,
.name = "lguest",

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