Re: [PATCH v2 11/43] KVM: Don't block+unblock when halt-polling is successful

[Sean Christopherson <seanjc@google.com>]

On Mon, Nov 29, 2021, Paolo Bonzini wrote:
> On 11/29/21 18:25, Sean Christopherson wrote:
> > If a posted interrupt arrives after KVM has done its final search through the vIRR,
> > but before avic_update_iommu_vcpu_affinity() is called, the posted interrupt will
> > be set in the vIRR without triggering a host IRQ to wake the vCPU via the GA log.
> > 
> > I.e. KVM is missing an equivalent to VMX's posted interrupt check for an outstanding
> > notification after switching to the wakeup vector.
> 
> BTW Maxim reported that it can break even without assigned devices.
> 
> > For now, the least awful approach is sadly to keep the vcpu_(un)blocking() hooks.
> 
> I agree that the hooks cannot be dropped but the bug is reproducible with
> this patch, where the hooks are still there.

...

> Still it does seem to be a race that happens when IS_RUNNING=true but
> vcpu->mode == OUTSIDE_GUEST_MODE.  This patch makes the race easier to
> trigger because it moves IS_RUNNING=false later.

Oh!  Any chance the bug only repros with preemption enabled?  That would explain
why I don't see problems, I'm pretty sure I've only run AVIC with a PREEMPT=n.

svm_vcpu_{un}blocking() are called with preemption enabled, and avic_set_running()
passes in vcpu->cpu.  If the vCPU is preempted and scheduled in on a different CPU,
avic_vcpu_load() will overwrite the vCPU's entry with the wrong CPU info.
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