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

[Paolo Bonzini]

On 11/29/21 19:55, Sean Christopherson wrote:
> > 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.

Me too.

> 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.

That would make a lot of sense.  avic_vcpu_load() can handle 
svm->avic_is_running = false, but avic_set_running still needs its body 
wrapped by preempt_disable/preempt_enable.

Fedora's kernel is CONFIG_PREEMPT_VOLUNTARY, but I know Maxim uses his 
own build so it would not surprise me if he used CONFIG_PREEMPT=y.

Paolo