Re: [RFC] KVM: Fix simultaneous NMIs

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On Mon, Sep 19, 2011 at 06:37:35PM +0300, Avi Kivity wrote:
> On 09/19/2011 06:22 PM, Marcelo Tosatti wrote:
> >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.
> 
> That gives a queue length of 3: one running nmi and nmi_pending = 2.

Increment through the same wrapper that will collapse the second and next, also
used by kvm_inject_nmi.

> >>   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?
> >
> 
> We have to, since we need a back-to-back nmi if the queue length > 1
> (including the running nmi).
> 
> -- 
> error compiling committee.c: too many arguments to function
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