Re: [RFC] KVM: Fix simultaneous NMIs

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Sep 19, 2011 at 06:09:39PM +0300, Avi Kivity wrote:
> On 09/19/2011 05:54 PM, Marcelo Tosatti wrote:
> >On Mon, Sep 19, 2011 at 05:30:27PM +0300, Avi Kivity wrote:
> >>  On 09/19/2011 04:54 PM, Marcelo Tosatti wrote:
> >>  >>   >>
> >>  >>   >>    Yes, due to NMI-blocked-by-STI.  A really touchy area.
> >>  >>   >And we don't need the window exit notification then? I don't understand
> >>  >>   >what nmi_in_progress is supposed to do here.
> >>  >>
> >>  >>   We need the window notification in both cases.  If we're recovering
> >>  >>   from STI, then we don't need to collapse NMIs.  If we're completing
> >>  >>   an NMI handler, then we do need to collapse NMIs (since the queue
> >>  >>   length is two, and we just completed one).
> >>  >
> >>  >I don't understand what is the point with nmi_in_progress, and the above
> >>  >hunk, either. Can't inject_nmi do:
> >>  >
> >>  >if (nmi_injected + atomic_read(nmi_pending)<   2)
> >>  >      atomic_inc(nmi_pending)
> >>  >
> >>  >Instead of collapsing somewhere else?
> >>
> >>  We could.  It's not atomic though - two threads executing in
> >>  parallel could raise the value to three.  Could do a cmpxchg loop
> >>  does an increment bounded to two.  I guess this is a lot clearer,
> >>  thanks.
> >>
> >>  >You'd also have to change
> >>  >nmi_injected handling in arch code so its value is not "hidden", in
> >>  >complete_interrupts().
> >>
> >>  Or maybe make raising nmi_injected not decrement nmi_pending.  So:
> >>
> >>    nmi_pending: total number of interrupts in queue
> >>    nmi_injected: of these, how many are currently being injected
> >>
> >>  yes?
> >
> >Yes, at the expense of decrementing on subarch code (which is fine,
> >apparently).
> >
> 
> Hm, we have no place to decrement. 

Decrement when setting nmi_injected = false, increment when setting
nmi_injected = true, in vmx/svm.c.

>  We need to do that when IRET
> executes, but we don't want to request an NMI window exit in the
> common case of nmi_pending = 1.

Do not enable nmi window if nmi_injected = true?