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Subject[PATCH 7/7] lguest: documentation pt VII: FIXMEs
From
Date
Documentation: The FIXMEs

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

---
Documentation/lguest/lguest.c | 12 ++++++++++++
drivers/char/hvc_lguest.c | 3 +++
drivers/lguest/interrupts_and_traps.c | 14 ++++++++++++++
drivers/lguest/io.c | 10 ++++++++++
drivers/lguest/lguest.c | 8 ++++++++
drivers/lguest/lguest_asm.S | 14 ++++++++++++++
drivers/lguest/page_tables.c | 5 +++++
drivers/lguest/segments.c | 4 ++++
drivers/net/lguest_net.c | 19 +++++++++++++++++++
9 files changed, 89 insertions(+)

===================================================================
--- a/Documentation/lguest/lguest.c
+++ b/Documentation/lguest/lguest.c
@@ -1536,3 +1536,15 @@ int main(int argc, char *argv[])
/* Finally, run the Guest. This doesn't return. */
run_guest(lguest_fd, &device_list);
}
+/*:*/
+
+/*M:999
+ * Mastery is done: you now know everything I do.
+ *
+ * But surely you have seen code, features and bugs in your wanderings which
+ * you now yearn to attack? That is the real game, and I look forward to you
+ * patching and forking lguest into the Your-Name-Here-visor.
+ *
+ * Farewell, and good coding!
+ * Rusty Russell.
+ */
===================================================================
--- a/drivers/char/hvc_lguest.c
+++ b/drivers/char/hvc_lguest.c
@@ -13,6 +13,9 @@
* functions.
:*/

+/*M:002 The console can be flooded: while the Guest is processing input the
+ * Host can send more. Buffering in the Host could alleviate this, but it is a
+ * difficult problem in general. :*/
/* Copyright (C) 2006 Rusty Russell, IBM Corporation
*
* This program is free software; you can redistribute it and/or modify
===================================================================
--- a/drivers/lguest/interrupts_and_traps.c
+++ b/drivers/lguest/interrupts_and_traps.c
@@ -231,6 +231,20 @@ static int direct_trap(const struct lgue
* go direct, of course 8) */
return idt_type(trap->a, trap->b) == 0xF;
}
+/*:*/
+
+/*M:005 The Guest has the ability to turn its interrupt gates into trap gates,
+ * if it is careful. The Host will let trap gates can go directly to the
+ * Guest, but the Guest needs the interrupts atomically disabled for an
+ * interrupt gate. It can do this by pointing the trap gate at instructions
+ * within noirq_start and noirq_end, where it can safely disable interrupts. */
+
+/*M:006 The Guests do not use the sysenter (fast system call) instruction,
+ * because it's hardcoded to enter privilege level 0 and so can't go direct.
+ * It's about twice as fast as the older "int 0x80" system call, so it might
+ * still be worthwhile to handle it in the Switcher and lcall down to the
+ * Guest. The sysenter semantics are hairy tho: search for that keyword in
+ * entry.S :*/

/*H:260 When we make traps go directly into the Guest, we need to make sure
* the kernel stack is valid (ie. mapped in the page tables). Otherwise, the
===================================================================
--- a/drivers/lguest/io.c
+++ b/drivers/lguest/io.c
@@ -553,6 +553,16 @@ void release_all_dma(struct lguest *lg)
up_read(&lg->mm->mmap_sem);
}

+/*M:007 We only return a single DMA buffer to the Launcher, but it would be
+ * more efficient to return a pointer to the entire array of DMA buffers, which
+ * it can cache and choose one whenever it wants.
+ *
+ * Currently the Launcher uses a write to /dev/lguest, and the return value is
+ * the address of the DMA structure with the interrupt number placed in
+ * dma->used_len. If we wanted to return the entire array, we need to return
+ * the address, array size and interrupt number: this seems to require an
+ * ioctl(). :*/
+
/*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,
===================================================================
--- a/drivers/lguest/lguest.c
+++ b/drivers/lguest/lguest.c
@@ -251,6 +251,14 @@ static void irq_enable(void)
{
lguest_data.irq_enabled = X86_EFLAGS_IF;
}
+/*:*/
+/*M:003 Note that we don't check for outstanding interrupts when we re-enable
+ * them (or when we unmask an interrupt). This seems to work for the moment,
+ * since interrupts are rare and we'll just get the interrupt on the next timer
+ * tick, but now we have CONFIG_NO_HZ, we should revisit this. One way
+ * would be to put the "irq_enabled" field in a page by itself, and have the
+ * Host write-protect it when an interrupt comes in when irqs are disabled.
+ * There will then be a page fault as soon as interrupts are re-enabled. :*/

/*G:034
* The Interrupt Descriptor Table (IDT).
===================================================================
--- a/drivers/lguest/lguest_asm.S
+++ b/drivers/lguest/lguest_asm.S
@@ -41,6 +41,20 @@ LGUEST_PATCH(pushf, movl lguest_data+LGU
.global lguest_noirq_start
.global lguest_noirq_end

+/*M:004 When the Host reflects a trap or injects an interrupt into the Guest,
+ * it sets the eflags interrupt bit on the stack based on
+ * lguest_data.irq_enabled, so the Guest iret logic does the right thing when
+ * restoring it. However, when the Host sets the Guest up for direct traps,
+ * such as system calls, the processor is the one to push eflags onto the
+ * stack, and the interrupt bit will be 1 (in reality, interrupts are always
+ * enabled in the Guest).
+ *
+ * This turns out to be harmless: the only trap which should happen under Linux
+ * with interrupts disabled is Page Fault (due to our lazy mapping of vmalloc
+ * regions), which has to be reflected through the Host anyway. If another
+ * trap *does* go off when interrupts are disabled, the Guest will panic, and
+ * we'll never get to this iret! :*/
+
/*G:045 There is one final paravirt_op that the Guest implements, and glancing
* at it you can see why I left it to last. It's *cool*! It's in *assembler*!
*
===================================================================
--- a/drivers/lguest/page_tables.c
+++ b/drivers/lguest/page_tables.c
@@ -14,6 +14,11 @@
#include <linux/percpu.h>
#include <asm/tlbflush.h>
#include "lg.h"
+
+/*M:008 We hold reference to pages, which prevents them from being swapped.
+ * It'd be nice to have a callback in the "struct mm_struct" when Linux wants
+ * to swap out. If we had this, and a shrinker callback to trim PTE pages, we
+ * could probably consider launching Guests as non-root. :*/

/*H:300
* The Page Table Code
===================================================================
--- a/drivers/lguest/segments.c
+++ b/drivers/lguest/segments.c
@@ -94,6 +94,10 @@ static void check_segment_use(struct lgu
|| lg->regs->ss / 8 == desc)
kill_guest(lg, "Removed live GDT entry %u", desc);
}
+/*:*/
+/*M:009 We wouldn't need to check for removal of in-use segments if we handled
+ * faults in the Switcher. However, it's probably not a worthwhile
+ * optimization. :*/

/*H:610 Once the GDT has been changed, we look through the changed entries and
* see if they're OK. If not, we'll call kill_guest() and the Guest will never
===================================================================
--- a/drivers/net/lguest_net.c
+++ b/drivers/net/lguest_net.c
@@ -34,6 +34,25 @@
#define SHARED_SIZE PAGE_SIZE
#define MAX_LANS 4
#define NUM_SKBS 8
+
+/*M:011 Network code master Jeff Garzik points out numerous shortcomings in
+ * this driver if it aspires to greatness.
+ *
+ * Firstly, it doesn't use "NAPI": the networking's New API, and is poorer for
+ * it. As he says "NAPI means system-wide load leveling, across multiple
+ * network interfaces. Lack of NAPI can mean competition at higher loads."
+ *
+ * He also points out that we don't implement set_mac_address, so users cannot
+ * change the devices hardware address. When I asked why one would want to:
+ * "Bonding, and situations where you /do/ want the MAC address to "leak" out
+ * of the host onto the wider net."
+ *
+ * Finally, he would like module unloading: "It is not unrealistic to think of
+ * [un|re|]loading the net support module in an lguest guest. And, adding
+ * module support makes the programmer more responsible, because they now have
+ * to learn to clean up after themselves. Any driver that cannot clean up
+ * after itself is an incomplete driver in my book."
+ :*/

/*D:530 The "struct lguestnet_info" contains all the information we need to
* know about the network device. */

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