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

[Sean Christopherson <seanjc@google.com>]

On Tue, Feb 04, 2025, Naveen N Rao wrote:
> 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?

Why?  All that would do is add complexity (taking two locks, or ensuring there
is no race when juggling locks), because if the guest is actually toggling AutoEOI
at a meaningful rate on multiple vCPUs, then there is going to be lock contention
regardless of which lock is taken.