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

[Avi Kivity <avi@redhat.com>]

On 02/03/2011 05:02 PM, Avi Kivity wrote:
> When we enable an NMI window, we ask for an IRET intercept, since
> the IRET re-enables NMIs.  However, the IRET intercept happens before
> the instruction executes, while the NMI window architecturally opens
> afterwards.
>
> To compensate for this mismatch, we only open the NMI window in the
> following exit, assuming that the IRET has by then executed; however,
> this assumption is not always correct; we may exit due to a host interrupt
> or page fault, without having executed the instruction.
>
> Fix by checking for forward progress by recording and comparing the IRET's
> rip.  This is somewhat of a hack, since an unchaging rip does not mean that
> no forward progress has been made, but is the simplest fix for now.
>

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?

-- 
error compiling committee.c: too many arguments to function