Re: [PATCH 1/2] KVM: Fix race between nmi injection and enabling nmi window

[Avi Kivity <avi@redhat.com>]

On 02/03/2011 05:11 PM, Jan Kiszka wrote:
> On 2011-02-03 16:02, Avi Kivity wrote:
> >  The interrupt injection logic looks something like
> >
> >    if an nmi is pending, and nmi injection allowed
> >      inject nmi
> >    if an nmi is pending
> >      request exit on nmi window
> >
> >  the problem is that "nmi is pending" can be set asynchronously by
> >  the PIT; if it happens to fire between the two if statements, we
> >  will request an nmi window even though nmi injection is allowed.  On
> >  SVM, this has disasterous results, since it causes eflags.TF to be
> >  set in random guest code.
>
> Good point. Fortunately never seen on production machines so far here
> (we have very moderate NMI rates).

I've never seen it either, except with ftrace enabled.  I wonder what 
the connection is.

> >
> >  The fix is simple; make nmi_pending asynchronous using the standard
>
> You mean synchronous, no?
>

Yes.

> >  vcpu->requests mechanism; this ensures the code above is completely
> >  synchronous wrt nmi_pending.
> >

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