[Lguest] 2.6.33 guest crash (was: Re: 2.6.32-rc5 guest crash)

[Rusty Russell]

On Mon, 15 Mar 2010 07:53:10 am Johannes Stezenbach wrote:
> On Sun, Mar 14, 2010 at 06:34:59PM +0100, I wrote:
> > 
> > Am I correct to assume that I can avoid the issue
> > by switching to a 64bit kernel (on host + guest)?
> 
> Silly question  ;-/
> 
> So what would be the real fix?

That patch broke Real Machines.  The real answer is actually to do some
more emulation in the host; I like lguest but I can't really justify many
lguest-specific hacks outside the lguest dirs.

There are a few patches needed to make Linus' latest work, I'll post them
soon.  But for this specific issue, how's this?

Subject: lguest: workaround cmpxchg8b_emu by ignoring cli in the guest.

It's only used by cmpxchg8b_emu (see db677ffa5f5a for the gory
details), and fixing that to be paravirt aware would be more work than
simply ignoring it (and AFAICT only help lguest).

(We can't emulate it properly: the popf which expects to restore interrupts
does not trap).

Signed-off-by: Rusty Russell <rusty at rustcorp.com.au>
Cc: Jeremy Fitzhardinge <jeremy at xensource.com>
Cc: virtualization at lists.osdl.org

diff --git a/drivers/lguest/x86/core.c b/drivers/lguest/x86/core.c
--- a/drivers/lguest/x86/core.c
+++ b/drivers/lguest/x86/core.c
@@ -288,6 +288,18 @@ static int emulate_insn(struct lg_cpu *c
 	insn = lgread(cpu, physaddr, u8);
 
 	/*
+	 * Around 2.6.33, the kernel started using an emulation for the
+	 * cmpxchg8b instruction in early boot on many configurations.  This
+	 * code isn't paravirtualized, and it tries to disable interrupts.
+	 * Ignore it, which will Mostly Work.
+	 */
+	if (insn == 0xfa) {
+		/* "cli", or Clear Interrupt Enable instruction.  Skip it. */ 
+		cpu->regs->eip++;
+		return 1;
+	}
+
+	/*
 	 * 0x66 is an "operand prefix".  It means it's using the upper 16 bits
 	 * of the eax register.
 	 */