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 05:13 PM, Jan Kiszka wrote:
>
>> I forgot them already.  What was that, exception during IRET?
> Exception during IRET or any instruction under the interrupt shadow will
> push the TF we set to step over this issue on the guest stack. We do not
> intercept all the possible exceptions, so we can leak TF. Moreover,
> multiplexing TF users is currently imperfect on AMD but, before fixing
> that, we have to think about the approach in general.

Thanks.  I think those are all solvable.  The key IMO is to take a state 
based approach to host bits - instead of setting or clearing a bit in 
response to an event, use the event as a trigger for recalculation of 
the bit's value.  This works for bits which have multiple uses, and for 
recovery from KVM_SET_*.  For guest bits which are needed by the host we 
also have a working approach - when the bit is overloaded, trap all 
instructions that can see it, as in CR0.TS.

It may take some work but I think we can achieve 100% accuracy without 
making the code unmaintainable.

>> 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.
>>
> Hope the above helps you to get on track, otherwise drop more questions.
> Also putting Joerg on CC (in the futile hope that the longer the CC list
> is, the lesser the pain becomes for each individual).

I think I got it.  And I also think we need to start documenting these 
invariants, to make it easier for people to see the method in all that 
chaos.

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