Re: [PATCH v2] KVM: Fix simultaneous NMIs

Avi Kivity
2011-09-20T16:24:06Z

On 09/20/2011 05:59 PM, Marcelo Tosatti wrote:
> On Tue, Sep 20, 2011 at 04:56:02PM +0300, Avi Kivity wrote:
> > On 09/20/2011 04:25 PM, Marcelo Tosatti wrote:
> > >>
> > >> @@ -2827,6 +2828,7 @@ static int kvm_vcpu_ioctl_x86_set_mce(struct kvm_vcpu *vcpu,
> > >> static void kvm_vcpu_ioctl_x86_get_vcpu_events(struct kvm_vcpu *vcpu,
> > >> struct kvm_vcpu_events *events)
> > >> {
> > >> + process_nmi(vcpu);
> > >> events->exception.injected =
> > >> vcpu->arch.exception.pending&&
> > >> !kvm_exception_is_soft(vcpu->arch.exception.nr);
> > >> @@ -2844,7 +2846,7 @@ static void kvm_vcpu_ioctl_x86_get_vcpu_events(struct kvm_vcpu *vcpu,
> > >> KVM_X86_SHADOW_INT_MOV_SS | KVM_X86_SHADOW_INT_STI);
> > >>
> > >> events->nmi.injected = vcpu->arch.nmi_injected;
> > >> - events->nmi.pending = vcpu->arch.nmi_pending;
> > >> + events->nmi.pending = vcpu->arch.nmi_pending != 0;
> > >> events->nmi.masked = kvm_x86_ops->get_nmi_mask(vcpu);
> > >> events->nmi.pad = 0;
> > >
> > >nmi_queued should also be saved and restored. Not sure if its necessary
> > >though.
> > >
> > >Should at least reset nmi_queued somewhere (set_vcpu_events?).
> >
> > Did you miss the call to process_nmi()?
>
> It transfers nmi_queued to nmi_pending with capping. What i mean is that
> upon system reset, nmi_queued (which refers to pre-reset system state)
> should be zeroed.

The is get_events(); for set_events(), process_nmi() does zero
nmi_queued() (and then we overwrite nmi_pending).

>
> > We do have a small issue. If we exit during NMI-blocked-by-STI and
> > nmi_pending == 2, then we lose the second interrupt. Should rarely
> > happen, since external interrupts never exit in that condition, but
> > it's a wart.
>
> And the above system reset case, you should be able to handle it by
> saving/restoring nmi_queued (so that QEMU can zero it in vcpu_reset).

We could just add a KVM_CAP (and flag) that extends nmi_pending from a
bool to a counter.

--
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