Re: [PATCH 1/3] KVM: x86: hyper-v: Convert synic_auto_eoi_used to an atomic

Naveen N Rao
2025-02-04T13:15:48Z

Hi Maxim,

On Mon, Feb 03, 2025 at 08:30:13PM -0500, Maxim Levitsky wrote:
> On Mon, 2025-02-03 at 22:33 +0530, Naveen N Rao (AMD) wrote:
> > diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> > index 6a6dd5a84f22..7a4554ea1d16 100644
> > --- a/arch/x86/kvm/hyperv.c
> > +++ b/arch/x86/kvm/hyperv.c
> > @@ -131,25 +131,18 @@ static void synic_update_vector(struct kvm_vcpu_hv_synic *synic,
> > if (auto_eoi_old == auto_eoi_new)
> > return;
> >
> > - if (!enable_apicv)
> > - return;
> > -
> > - down_write(&vcpu->kvm->arch.apicv_update_lock);
> > -
> > if (auto_eoi_new)
> > - hv->synic_auto_eoi_used++;
> > + atomic_inc(&hv->synic_auto_eoi_used);
> > else
> > - hv->synic_auto_eoi_used--;
> > + atomic_dec(&hv->synic_auto_eoi_used);
> >
> > /*
> > * Inhibit APICv if any vCPU is using SynIC's AutoEOI, which relies on
> > * the hypervisor to manually inject IRQs.
> > */
> > - __kvm_set_or_clear_apicv_inhibit(vcpu->kvm,
> > - APICV_INHIBIT_REASON_HYPERV,
> > - !!hv->synic_auto_eoi_used);
> > -
> > - up_write(&vcpu->kvm->arch.apicv_update_lock);
> > + kvm_set_or_clear_apicv_inhibit(vcpu->kvm,
> > + APICV_INHIBIT_REASON_HYPERV,
> > + !!atomic_read(&hv->synic_auto_eoi_used));
>
> Hi,
>
> This introduces a race, because there is a race window between
> the moment we read hv->synic_auto_eoi_used, and decide to set/clear the inhibit.
>
> After we read hv->synic_auto_eoi_used, but before we call the kvm_set_or_clear_apicv_inhibit,
> other core might also run synic_update_vector and change hv->synic_auto_eoi_used,
> finish setting the inhibit in kvm_set_or_clear_apicv_inhibit,
> and only then we will call kvm_set_or_clear_apicv_inhibit with the stale value of hv->synic_auto_eoi_used and clear it.

Ah, indeed. Thanks for the explanation.

I wonder if we can switch to using kvm_hv->hv_lock in place of
apicv_update_lock. That lock is already used to guard updates to
partition-wide MSRs in kvm_hv_set_msr_common(). So, that might be ok
too?

Thanks,
Naveen