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From: Jeremy Fitzhardinge on 30 Mar 2010 01:00
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(a)rustcorp.com.au>
> Cc: Jeremy Fitzhardinge<jeremy(a)xensource.com>
> Cc: virtualization(a)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.
> */
>
>

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