linux-kernel - Re: [Lguest] 2.6.33 guest crash

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Date:	Mon, 29 Mar 2010 21:51:38 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Johannes Stezenbach <js@...21.net>, lguest@...abs.org,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>,
	Gaurav Kukreja <mailme.gaurav@...il.com>
Subject: Re: [Lguest]  2.6.33 guest crash

On 03/29/2010 09:27 PM, Rusty Russell wrote:
> 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).
>    

Why isn't the cli getting paravirtualized?

     J

> Signed-off-by: Rusty Russell<rusty@...tcorp.com.au>
> Cc: Jeremy Fitzhardinge<jeremy@...source.com>
> Cc: virtualization@...ts.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.
>   	 */
>
>    

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