Re: [PATCH 1/1] Disable GUEST_INTR_STATE_STI flag before injecting NMI to guest on VMX

[Avi Kivity <avi@redhat.com>]

  On 08/27/2010 02:06 PM, Jan Kiszka wrote:
> Avi Kivity wrote:
>>    On 08/27/2010 11:39 AM, Jan Kiszka wrote:
>>> AMD does not differentiate between MOV-SS and STI interrupt shadows.
>>>
>>> But AMD has its own NMI problems as it does not allow to trap after
>>> IRET-from-NMI and requires magic dances which are partly broken in KVM.
>>> I'm leaning towards NMI window emulation via the workaround we use for
>>> older Intel CPUs without NMI window trapping as well.
>> Can you elaborate?
> Basically the issues you found regarding our single-step-based
> workaround.

I forgot them already.  What was that, exception during IRET?

> Moreover, we cannot easily prevent that TF set by the NMI
> code leaks onto the guest's stack.
>
> Jörg and I stuck heads together about this during LinuxCon. We came to
> the conclusion that we either have to emulate the instruction that
> delays NMIs (ie. _every_ possible instruction for the interrupt shadow
> case) or fall back to the VMX workaround based on interrupt window
> trapping and an emergency timeout (much simpler, but not really correct).

I'd really like to avoid the timer.  But I forgot all the details around 
this, I'll have to re-learn them so I can actually compare the two options.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.