Re: [PATCH 2/2] KVM: SVM: check for progress after IRET interception

[Avi Kivity <avi@redhat.com>]

On 02/03/2011 05:21 PM, Jan Kiszka wrote:
> >
> >  So what would be a better fix?  We could unconditionally single step on
> >  iret_interception() which would fix the problem at the cost of making
> >  NMIs less efficient (three exits instead of two).  We could emulate the
> >  IRET (doubling kvm's code and likely slower, and certainly buggier, than
> >  the first option).  Alternatively, can anyone think of a reliable way to
> >  make sure forward progress has been made?
>
> Joerg and I discussed this a few times, I think last on the KVM forum.
> It's really tricky and we found no option without limitations.
> Single-stepping, e.g., already pollutes the guest state (if an exception
> is taken without prior vmexit).

We could enable all intercepts when single stepping.  if we get a 
single-step #DB, we clear nmi blocking and are happy.  If we get another 
exception, or a non-single-step #DB, we re-enable the IRET intercept, 
clear single-step, re-inject the exception, and let the guest continue.

> I don't recall all alternatives, but a vmexit-saving one was (IIRC) to
> fall back to an interrupt window without IRET interception, likely
> augmented with some break-out timer like we do for oldish, vnmi-lacking
> Intels.

What's an interrupt window without IRET interception?

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