Re: [PATCH 3/3] KVM: x86: Decouple APICv activation state from apicv_inhibit_reasons

Sean Christopherson
2025-02-11T16:37:19Z

On Tue, Feb 11, 2025, Naveen N Rao wrote:
> On Wed, Feb 05, 2025 at 12:36:21PM +0100, Paolo Bonzini wrote:
> I haven't analyzed this yet, but moving apicv_irq_window into a separate
> cacheline is improving the performance in my tests by ~7 to 8%:
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 9e3465e70a0a..d8a40ac49226 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1355,6 +1355,9 @@ struct kvm_arch {
> struct kvm_ioapic *vioapic;
> struct kvm_pit *vpit;
> atomic_t vapics_in_nmi_mode;
> +
> + atomic_t apicv_irq_window;
> +
> struct mutex apic_map_lock;
> struct kvm_apic_map __rcu *apic_map;
> atomic_t apic_map_dirty;
> @@ -1365,7 +1368,6 @@ struct kvm_arch {
> /* Protects apicv_inhibit_reasons */
> struct rw_semaphore apicv_update_lock;
> unsigned long apicv_inhibit_reasons;
> - atomic_t apicv_irq_window;
>
> gpa_t wall_clock;
>
>
> I chose that spot before apic_map_lock simply because there was a 4 byte
> hole there. This happens to also help performance in the AVIC disabled
> case by a few percentage points (rather, restores the performance in the
> AVIC disabled case).
>
> Before this change, I was trying to see if we could entirely elide the
> rwsem read lock in the specific scenario we are seeing the bottleneck.
> That is, instead of checking for any other inhibit being set, can we
> specifically test for PIT_REINJ while setting the IRQWIN inhibit? Then,
> update the inhibit change logic if PIT_REINJ is cleared to re-check the
> irq window count.
>
> There's probably a race here somewhere, but FWIW, along with the above
> change to 'struct kvm_arch', this helps improve performance by a few
> more percentage points helping close the gap to within 2% of the AVIC
> disabled case.

I suspect the issue is that apicv_inhibit_reasons is in the same cache line. That
field is read on at least every entry

/*
* Assert that vCPU vs. VM APICv state is consistent. An APICv
* update must kick and wait for all vCPUs before toggling the
* per-VM state, and responding vCPUs must wait for the update
* to complete before servicing KVM_REQ_APICV_UPDATE.
*/
WARN_ON_ONCE((kvm_vcpu_apicv_activated(vcpu) != kvm_vcpu_apicv_active(vcpu)) &&
(kvm_get_apic_mode(vcpu) != LAPIC_MODE_DISABLED));

and when opening an IRQ window in svm_set_vintr()

WARN_ON(kvm_vcpu_apicv_activated(&svm->vcpu));

and when handling emulated APIC MMIO in kvm_mmu_faultin_pfn():

/*
* If the APIC access page exists but is disabled, go directly
* to emulation without caching the MMIO access or creating a
* MMIO SPTE. That way the cache doesn't need to be purged
* when the AVIC is re-enabled.
*/
if (!kvm_apicv_activated(vcpu->kvm))
return RET_PF_EMULATE;

Hmm, now that I think about it, lack of emulated MMIO caching that might explain
the 2% gap. Do you see the same gap if the guest is using x2APIC?