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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-02-03 16:30, Avi Kivity wrote:
> 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.

Yes, I once explored this direction. It should be feasible (though
complex), need to check again why I suspended the effort.

> 
>> 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?

I don't the details, but I thought you could get something like an
interrupt-window-open interception by (fake-)injecting an IRQ and
intercepting on VIRQ acceptance. That will not work if returning to and
staying in irq-disabled guest code, therefore the timeout, but it should
be most efficient (specifically if the guest uses NMIs for things like
perf).

Jan

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