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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Feb 03, 2011 at 05:07:33PM +0200, Avi Kivity wrote:
> 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.
> >

Looks good.

> 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?

Is there other negative impact of the RIP hack than NMI being delayed?