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
