lguest: documentation II: Guest

Documentation: The Guest

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 41 insertions, 16 deletions

diff --git a/drivers/lguest/lguest_asm.S b/drivers/lguest/lguest_asm.S
index a3dbf22..3126ae9 100644
--- a/drivers/lguest/lguest_asm.S
+++ b/drivers/lguest/lguest_asm.S
@@ -4,15 +4,15 @@
 #include <asm/thread_info.h>
 #include <asm/processor-flags.h>
 
-/*
- * This is where we begin: we have a magic signature which the launcher looks
- * for.  The plan is that the Linux boot protocol will be extended with a
+/*G:020 This is where we begin: we have a magic signature which the launcher
+ * looks for.  The plan is that the Linux boot protocol will be extended with a
  * "platform type" field which will guide us here from the normal entry point,
- * but for the moment this suffices.  We pass the virtual address of the boot
- * info to lguest_init().
+ * but for the moment this suffices.  The normal boot code uses %esi for the
+ * boot header, so we do too.  We convert it to a virtual address by adding
+ * PAGE_OFFSET, and hand it to lguest_init() as its argument (ie. %eax).
  *
- * We put it in .init.text will be discarded after boot.
- */
+ * The .section line puts this code in .init.text so it will be discarded after
+ * boot. */
 .section .init.text, "ax", @progbits
 .ascii "GenuineLguest"
 	/* Set up initial stack. */
@@ -21,7 +21,9 @@
 	addl $__PAGE_OFFSET, %eax
 	jmp lguest_init
 
-/* The templates for inline patching. */
+/*G:055 We create a macro which puts the assembler code between lgstart_ and
+ * lgend_ markers.  These templates end up in the .init.text section, so they
+ * are discarded after boot. */
 #define LGUEST_PATCH(name, insns...)			\
 	lgstart_##name:	insns; lgend_##name:;		\
 	.globl lgstart_##name; .globl lgend_##name
@@ -30,24 +32,47 @@ LGUEST_PATCH(cli, movl $0, lguest_data+LGUEST_DATA_irq_enabled)
 LGUEST_PATCH(sti, movl $X86_EFLAGS_IF, lguest_data+LGUEST_DATA_irq_enabled)
 LGUEST_PATCH(popf, movl %eax, lguest_data+LGUEST_DATA_irq_enabled)
 LGUEST_PATCH(pushf, movl lguest_data+LGUEST_DATA_irq_enabled, %eax)
+/*:*/
 
 .text
 /* These demark the EIP range where host should never deliver interrupts. */
 .global lguest_noirq_start
 .global lguest_noirq_end
 
-/*
- * We move eflags word to lguest_data.irq_enabled to restore interrupt state.
- * For page faults, gpfs and virtual interrupts, the hypervisor has saved
- * eflags manually, otherwise it was delivered directly and so eflags reflects
- * the real machine IF state, ie. interrupts on.  Since the kernel always dies
- * if it takes such a trap with interrupts disabled anyway, turning interrupts
- * back on unconditionally here is OK.
- */
+/*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*!
+ *
+ * The "iret" instruction is used to return from an interrupt or trap.  The
+ * stack looks like this:
+ *   old address
+ *   old code segment & privilege level
+ *   old processor flags ("eflags")
+ *
+ * The "iret" instruction pops those values off the stack and restores them all
+ * at once.  The only problem is that eflags includes the Interrupt Flag which
+ * the Guest can't change: the CPU will simply ignore it when we do an "iret".
+ * So we have to copy eflags from the stack to lguest_data.irq_enabled before
+ * we do the "iret".
+ *
+ * There are two problems with this: firstly, we need to use a register to do
+ * the copy and secondly, the whole thing needs to be atomic.  The first
+ * problem is easy to solve: push %eax on the stack so we can use it, and then
+ * restore it at the end just before the real "iret".
+ *
+ * The second is harder: copying eflags to lguest_data.irq_enabled will turn
+ * interrupts on before we're finished, so we could be interrupted before we
+ * return to userspace or wherever.  Our solution to this is to surround the
+ * code with lguest_noirq_start: and lguest_noirq_end: labels.  We tell the
+ * Host that it is *never* to interrupt us there, even if interrupts seem to be
+ * enabled. */
 ENTRY(lguest_iret)
 	pushl	%eax
 	movl	12(%esp), %eax
 lguest_noirq_start:
+	/* Note the %ss: segment prefix here.  Normal data accesses use the
+	 * "ds" segment, but that will have already been restored for whatever
+	 * we're returning to (such as userspace): we can't trust it.  The %ss:
+	 * prefix makes sure we use the stack segment, which is still valid. */
 	movl	%eax,%ss:lguest_data+LGUEST_DATA_irq_enabled
 	popl	%eax
 	iret