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

[Johannes Stezenbach <js@sig21.net>]

Hi Rusty,

On Tue, Mar 30, 2010 at 02:57:14PM +1030, Rusty Russell wrote:
> 
> 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@rustcorp.com.au>
> Cc: Jeremy Fitzhardinge <jeremy@xensource.com>
> Cc: virtualization@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.
>  	 */
> 

I just tested this patch with 2.6.34-rc4 (as both host and guest),
it seems to work fine.


Thanks,
Johannes