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Subject[PATCH 6/7] lguest: documentation pt VI: Switcher
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Date
Documentation: The Switcher

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

---
drivers/lguest/core.c | 51 +++++++-
drivers/lguest/switcher.S | 271 ++++++++++++++++++++++++++++++++++++++-------
2 files changed, 276 insertions(+), 46 deletions(-)

===================================================================
--- a/drivers/lguest/core.c
+++ b/drivers/lguest/core.c
@@ -394,46 +394,89 @@ static void set_ts(void)
write_cr0(cr0|8);
}

+/*S:010
+ * We are getting close to the Switcher.
+ *
+ * Remember that each CPU has two pages which are visible to the Guest when it
+ * runs on that CPU. This has to contain the state for that Guest: we copy the
+ * state in just before we run the Guest.
+ *
+ * Each Guest has "changed" flags which indicate what has changed in the Guest
+ * since it last ran. We saw this set in interrupts_and_traps.c and
+ * segments.c.
+ */
static void copy_in_guest_info(struct lguest *lg, struct lguest_pages *pages)
{
+ /* Copying all this data can be quite expensive. We usually run the
+ * same Guest we ran last time (and that Guest hasn't run anywhere else
+ * meanwhile). If that's not the case, we pretend everything in the
+ * Guest has changed. */
if (__get_cpu_var(last_guest) != lg || lg->last_pages != pages) {
__get_cpu_var(last_guest) = lg;
lg->last_pages = pages;
lg->changed = CHANGED_ALL;
}

- /* These are pretty cheap, so we do them unconditionally. */
+ /* These copies are pretty cheap, so we do them unconditionally: */
+ /* Save the current Host top-level page directory. */
pages->state.host_cr3 = __pa(current->mm->pgd);
+ /* Set up the Guest's page tables to see this CPU's pages (and no
+ * other CPU's pages). */
map_switcher_in_guest(lg, pages);
+ /* Set up the two "TSS" members which tell the CPU what stack to use
+ * for traps which do directly into the Guest (ie. traps at privilege
+ * level 1). */
pages->state.guest_tss.esp1 = lg->esp1;
pages->state.guest_tss.ss1 = lg->ss1;

- /* Copy direct trap entries. */
+ /* Copy direct-to-Guest trap entries. */
if (lg->changed & CHANGED_IDT)
copy_traps(lg, pages->state.guest_idt, default_idt_entries);

- /* Copy all GDT entries but the TSS. */
+ /* Copy all GDT entries which the Guest can change. */
if (lg->changed & CHANGED_GDT)
copy_gdt(lg, pages->state.guest_gdt);
/* If only the TLS entries have changed, copy them. */
else if (lg->changed & CHANGED_GDT_TLS)
copy_gdt_tls(lg, pages->state.guest_gdt);

+ /* Mark the Guest as unchanged for next time. */
lg->changed = 0;
}

+/* Finally: the code to actually call into the Switcher to run the Guest. */
static void run_guest_once(struct lguest *lg, struct lguest_pages *pages)
{
+ /* This is a dummy value we need for GCC's sake. */
unsigned int clobber;

+ /* Copy the guest-specific information into this CPU's "struct
+ * lguest_pages". */
copy_in_guest_info(lg, pages);

- /* Put eflags on stack, lcall does rest: suitable for iret return. */
+ /* Now: we push the "eflags" register on the stack, then do an "lcall".
+ * This is how we change from using the kernel code segment to using
+ * the dedicated lguest code segment, as well as jumping into the
+ * Switcher.
+ *
+ * The lcall also pushes the old code segment (KERNEL_CS) onto the
+ * stack, then the address of this call. This stack layout happens to
+ * exactly match the stack of an interrupt... */
asm volatile("pushf; lcall *lguest_entry"
+ /* This is how we tell GCC that %eax ("a") and %ebx ("b")
+ * are changed by this routine. The "=" means output. */
: "=a"(clobber), "=b"(clobber)
+ /* %eax contains the pages pointer. ("0" refers to the
+ * 0-th argument above, ie "a"). %ebx contains the
+ * physical address of the Guest's top-level page
+ * directory. */
: "0"(pages), "1"(__pa(lg->pgdirs[lg->pgdidx].pgdir))
+ /* We tell gcc that all these registers could change,
+ * which means we don't have to save and restore them in
+ * the Switcher. */
: "memory", "%edx", "%ecx", "%edi", "%esi");
}
+/*:*/

/*H:030 Let's jump straight to the the main loop which runs the Guest.
* Remember, this is called by the Launcher reading /dev/lguest, and we keep
===================================================================
--- a/drivers/lguest/switcher.S
+++ b/drivers/lguest/switcher.S
@@ -6,41 +6,131 @@
* are feeling invigorated and refreshed then the next, more challenging stage
* can be found in "make Guest". :*/

+/*S:100
+ * Welcome to the Switcher itself!
+ *
+ * This file contains the low-level code which changes the CPU to run the Guest
+ * code, and returns to the Host when something happens. Understand this, and
+ * you understand the heart of our journey.
+ *
+ * Because this is in assembler rather than C, our tale switches from prose to
+ * verse. First I tried limericks:
+ *
+ * There once was an eax reg,
+ * To which our pointer was fed,
+ * It needed an add,
+ * Which asm-offsets.h had
+ * But this limerick is hurting my head.
+ *
+ * Next I tried haikus, but fitting the required reference to the seasons in
+ * every stanza was quickly becoming tiresome:
+ *
+ * The %eax reg
+ * Holds "struct lguest_pages" now:
+ * Cherry blossoms fall.
+ *
+ * Then I started with Heroic Verse, but the rhyming requirement leeched away
+ * the content density and led to some uniquely awful oblique rhymes:
+ *
+ * These constants are coming from struct offsets
+ * For use within the asm switcher text.
+ *
+ * Finally, I settled for something between heroic hexameter, and normal prose
+ * with inappropriate linebreaks. Anyway, it aint no Shakespeare.
+ */
+
+// Not all kernel headers work from assembler
+// But these ones are needed: the ENTRY() define
+// And constants extracted from struct offsets
+// To avoid magic numbers and breakage:
+// Should they change the compiler can't save us
+// Down here in the depths of assembler code.
#include <linux/linkage.h>
#include <asm/asm-offsets.h>
#include "lg.h"

+// We mark the start of the code to copy
+// It's placed in .text tho it's never run here
+// You'll see the trick macro at the end
+// Which interleaves data and text to effect.
.text
ENTRY(start_switcher_text)

-/* %eax points to lguest pages for this CPU. %ebx contains cr3 value.
- All normal registers can be clobbered! */
+// When we reach switch_to_guest we have just left
+// The safe and comforting shores of C code
+// %eax has the "struct lguest_pages" to use
+// Where we save state and still see it from the Guest
+// And %ebx holds the Guest shadow pagetable:
+// Once set we have truly left Host behind.
ENTRY(switch_to_guest)
- /* Save host segments on host stack. */
+ // We told gcc all its regs could fade,
+ // Clobbered by our journey into the Guest
+ // We could have saved them, if we tried
+ // But time is our master and cycles count.
+
+ // Segment registers must be saved for the Host
+ // We push them on the Host stack for later
pushl %es
pushl %ds
pushl %gs
pushl %fs
- /* With CONFIG_FRAME_POINTER, gcc doesn't let us clobber this! */
+ // But the compiler is fickle, and heeds
+ // No warning of %ebp clobbers
+ // When frame pointers are used. That register
+ // Must be saved and restored or chaos strikes.
pushl %ebp
- /* Save host stack. */
+ // The Host's stack is done, now save it away
+ // In our "struct lguest_pages" at offset
+ // Distilled into asm-offsets.h
movl %esp, LGUEST_PAGES_host_sp(%eax)
- /* Switch to guest stack: if we get NMI we expect to be there. */
+
+ // All saved and there's now five steps before us:
+ // Stack, GDT, IDT, TSS
+ // And last of all the page tables are flipped.
+
+ // Yet beware that our stack pointer must be
+ // Always valid lest an NMI hits
+ // %edx does the duty here as we juggle
+ // %eax is lguest_pages: our stack lies within.
movl %eax, %edx
addl $LGUEST_PAGES_regs, %edx
movl %edx, %esp
- /* Switch to guest's GDT, IDT. */
+
+ // The Guest's GDT we so carefully
+ // Placed in the "struct lguest_pages" before
lgdt LGUEST_PAGES_guest_gdt_desc(%eax)
+
+ // The Guest's IDT we did partially
+ // Move to the "struct lguest_pages" as well.
lidt LGUEST_PAGES_guest_idt_desc(%eax)
- /* Switch to guest's TSS while GDT still writable. */
+
+ // The TSS entry which controls traps
+ // Must be loaded up with "ltr" now:
+ // For after we switch over our page tables
+ // It (as the rest) will be writable no more.
+ // (The GDT entry TSS needs
+ // Changes type when we load it: damn Intel!)
movl $(GDT_ENTRY_TSS*8), %edx
ltr %dx
- /* Set host's TSS GDT entry to available (clear byte 5 bit 2). */
+
+ // Look back now, before we take this last step!
+ // The Host's TSS entry was also marked used;
+ // Let's clear it again, ere we return.
+ // The GDT descriptor of the Host
+ // Points to the table after two "size" bytes
movl (LGUEST_PAGES_host_gdt_desc+2)(%eax), %edx
+ // Clear the type field of "used" (byte 5, bit 2)
andb $0xFD, (GDT_ENTRY_TSS*8 + 5)(%edx)
- /* Switch to guest page tables: lguest_pages->state now read-only. */
+
+ // Once our page table's switched, the Guest is live!
+ // The Host fades as we run this final step.
+ // Our "struct lguest_pages" is now read-only.
movl %ebx, %cr3
- /* Restore guest regs */
+
+ // The page table change did one tricky thing:
+ // The Guest's register page has been mapped
+ // Writable onto our %esp (stack) --
+ // We can simply pop off all Guest regs.
popl %ebx
popl %ecx
popl %edx
@@ -52,12 +142,27 @@ ENTRY(switch_to_guest)
popl %fs
popl %ds
popl %es
- /* Skip error code and trap number */
+
+ // Near the base of the stack lurk two strange fields
+ // Which we fill as we exit the Guest
+ // These are the trap number and its error
+ // We can simply step past them on our way.
addl $8, %esp
+
+ // The last five stack slots hold return address
+ // And everything needed to change privilege
+ // Into the Guest privilege level of 1,
+ // And the stack where the Guest had last left it.
+ // Interrupts are turned back on: we are Guest.
iret

+// There are two paths where we switch to the Host
+// So we put the routine in a macro.
+// We are on our way home, back to the Host
+// Interrupted out of the Guest, we come here.
#define SWITCH_TO_HOST \
- /* Save guest state */ \
+ /* We save the Guest state: all registers first \
+ * Laid out just as "struct lguest_regs" defines */ \
pushl %es; \
pushl %ds; \
pushl %fs; \
@@ -69,58 +174,119 @@ ENTRY(switch_to_guest)
pushl %edx; \
pushl %ecx; \
pushl %ebx; \
- /* Load lguest ds segment for convenience. */ \
+ /* Our stack and our code are using segments \
+ * Set in the TSS and IDT \
+ * Yet if we were to touch data we'd use \
+ * Whatever data segment the Guest had. \
+ * Load the lguest ds segment for now. */ \
movl $(LGUEST_DS), %eax; \
movl %eax, %ds; \
- /* Figure out where we are, based on stack (at top of regs). */ \
+ /* So where are we? Which CPU, which struct? \
+ * The stack is our clue: our TSS sets \
+ * It at the end of "struct lguest_pages" \
+ * And we then pushed and pushed and pushed Guest regs: \
+ * Now stack points atop the "struct lguest_regs". \
+ * Subtract that offset, and we find our struct. */ \
movl %esp, %eax; \
subl $LGUEST_PAGES_regs, %eax; \
- /* Put trap number in %ebx before we switch cr3 and lose it. */ \
+ /* Save our trap number: the switch will obscure it \
+ * (The Guest regs are not mapped here in the Host) \
+ * %ebx holds it safe for deliver_to_host */ \
movl LGUEST_PAGES_regs_trapnum(%eax), %ebx; \
- /* Switch to host page tables (host GDT, IDT and stack are in host \
- mem, so need this first) */ \
+ /* The Host GDT, IDT and stack! \
+ * All these lie safely hidden from the Guest: \
+ * We must return to the Host page tables \
+ * (Hence that was saved in struct lguest_pages) */ \
movl LGUEST_PAGES_host_cr3(%eax), %edx; \
movl %edx, %cr3; \
- /* Set guest's TSS to available (clear byte 5 bit 2). */ \
+ /* As before, when we looked back at the Host \
+ * As we left and marked TSS unused \
+ * So must we now for the Guest left behind. */ \
andb $0xFD, (LGUEST_PAGES_guest_gdt+GDT_ENTRY_TSS*8+5)(%eax); \
- /* Switch to host's GDT & IDT. */ \
+ /* Switch to Host's GDT, IDT. */ \
lgdt LGUEST_PAGES_host_gdt_desc(%eax); \
lidt LGUEST_PAGES_host_idt_desc(%eax); \
- /* Switch to host's stack. */ \
+ /* Restore the Host's stack where it's saved regs lie */ \
movl LGUEST_PAGES_host_sp(%eax), %esp; \
- /* Switch to host's TSS */ \
+ /* Last the TSS: our Host is complete */ \
movl $(GDT_ENTRY_TSS*8), %edx; \
ltr %dx; \
+ /* Restore now the regs saved right at the first. */ \
popl %ebp; \
popl %fs; \
popl %gs; \
popl %ds; \
popl %es

-/* Return to run_guest_once. */
+// Here's where we come when the Guest has just trapped:
+// (Which trap we'll see has been pushed on the stack).
+// We need only switch back, and the Host will decode
+// Why we came home, and what needs to be done.
return_to_host:
SWITCH_TO_HOST
iret

+// An interrupt, with some cause external
+// Has ajerked us rudely from the Guest's code
+// Again we must return home to the Host
deliver_to_host:
SWITCH_TO_HOST
- /* Decode IDT and jump to hosts' irq handler. When that does iret, it
- * will return to run_guest_once. This is a feature. */
+ // But now we must go home via that place
+ // Where that interrupt was supposed to go
+ // Had we not been ensconced, running the Guest.
+ // Here we see the cleverness of our stack:
+ // The Host stack is formed like an interrupt
+ // With EIP, CS and EFLAGS layered.
+ // Interrupt handlers end with "iret"
+ // And that will take us home at long long last.
+
+ // But first we must find the handler to call!
+ // The IDT descriptor for the Host
+ // Has two bytes for size, and four for address:
+ // %edx will hold it for us for now.
movl (LGUEST_PAGES_host_idt_desc+2)(%eax), %edx
+ // We now know the table address we need,
+ // And saved the trap's number inside %ebx.
+ // Yet the pointer to the handler is smeared
+ // Across the bits of the table entry.
+ // What oracle can tell us how to extract
+ // From such a convoluted encoding?
+ // I consulted gcc, and it gave
+ // These instructions, which I gladly credit:
leal (%edx,%ebx,8), %eax
movzwl (%eax),%edx
movl 4(%eax), %eax
xorw %ax, %ax
orl %eax, %edx
+ // Now the address of the handler's in %edx
+ // We call it now: its "iret" takes us home.
jmp *%edx

-/* Real hardware interrupts are delivered straight to the host. Others
- cause us to return to run_guest_once so it can decide what to do. Note
- that some of these are overridden by the guest to deliver directly, and
- never enter here (see load_guest_idt_entry). */
+// Every interrupt can come to us here
+// But we must truly tell each apart.
+// They number two hundred and fifty six
+// And each must land in a different spot,
+// Push its number on stack, and join the stream.
+
+// And worse, a mere six of the traps stand apart
+// And push on their stack an addition:
+// An error number, thirty two bits long
+// So we punish the other two fifty
+// And make them push a zero so they match.
+
+// Yet two fifty six entries is long
+// And all will look most the same as the last
+// So we create a macro which can make
+// As many entries as we need to fill.
+
+// Note the change to .data then .text:
+// We plant the address of each entry
+// Into a (data) table for the Host
+// To know where each Guest interrupt should go.
.macro IRQ_STUB N TARGET
.data; .long 1f; .text; 1:
- /* Make an error number for most traps, which don't have one. */
+ // Trap eight, ten through fourteen and seventeen
+ // Supply an error number. Else zero.
.if (\N <> 8) && (\N < 10 || \N > 14) && (\N <> 17)
pushl $0
.endif
@@ -129,6 +295,8 @@ deliver_to_host:
ALIGN
.endm

+// This macro creates numerous entries
+// Using GAS macros which out-power C's.
.macro IRQ_STUBS FIRST LAST TARGET
irq=\FIRST
.rept \LAST-\FIRST+1
@@ -137,24 +305,43 @@ deliver_to_host:
.endr
.endm

-/* We intercept every interrupt, because we may need to switch back to
- * host. Unfortunately we can't tell them apart except by entry
- * point, so we need 256 entry points.
- */
+// Here's the marker for our pointer table
+// Laid in the data section just before
+// Each macro places the address of code
+// Forming an array: each one points to text
+// Which handles interrupt in its turn.
.data
.global default_idt_entries
default_idt_entries:
.text
- IRQ_STUBS 0 1 return_to_host /* First two traps */
- IRQ_STUB 2 handle_nmi /* NMI */
- IRQ_STUBS 3 31 return_to_host /* Rest of traps */
- IRQ_STUBS 32 127 deliver_to_host /* Real interrupts */
- IRQ_STUB 128 return_to_host /* System call (overridden) */
- IRQ_STUBS 129 255 deliver_to_host /* Other real interrupts */
-
-/* We ignore NMI and return. */
+ // The first two traps go straight back to the Host
+ IRQ_STUBS 0 1 return_to_host
+ // We'll say nothing, yet, about NMI
+ IRQ_STUB 2 handle_nmi
+ // Other traps also return to the Host
+ IRQ_STUBS 3 31 return_to_host
+ // All interrupts go via their handlers
+ IRQ_STUBS 32 127 deliver_to_host
+ // 'Cept system calls coming from userspace
+ // Are to go to the Guest, never the Host.
+ IRQ_STUB 128 return_to_host
+ IRQ_STUBS 129 255 deliver_to_host
+
+// The NMI, what a fabulous beast
+// Which swoops in and stops us no matter that
+// We're suspended between heaven and hell,
+// (Or more likely between the Host and Guest)
+// When in it comes! We are dazed and confused
+// So we do the simplest thing which one can.
+// Though we've pushed the trap number and zero
+// We discard them, return, and hope we live.
handle_nmi:
addl $8, %esp
iret

+// We are done; all that's left is Mastery
+// And "make Mastery" is a journey long
+// Designed to make your fingers itch to code.
+
+// Here ends the text, the file and poem.
ENTRY(end_switcher_text)

-
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