Li RongQing <lirongqing@xxxxxxxxx> writes:
> merge pv_eoi_get_pending and pv_eoi_clr_pending into a single
> function pv_eoi_test_and_clear_pending, which returns and clear
> the value of the pending bit.
>
> and clear pv eoi pending bit only when it is set, to avoid calling
> pv_eoi_put_user(), this can speed about 300 nsec on AMD EPYC most
> of the time
>
> Suggested-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Signed-off-by: Li RongQing <lirongqing@xxxxxxxxx>
> ---
> diff v2: merge as pv_eoi_test_and_clear_pending
> diff v3: remove printk with a new patch
> arch/x86/kvm/lapic.c | 39 ++++++++++++++++++---------------------
> 1 files changed, 18 insertions(+), 21 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 752c48e..9c3b1b3 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -673,15 +673,6 @@ static inline bool pv_eoi_enabled(struct kvm_vcpu *vcpu)
> return vcpu->arch.pv_eoi.msr_val & KVM_MSR_ENABLED;
> }
>
> -static bool pv_eoi_get_pending(struct kvm_vcpu *vcpu)
> -{
> - u8 val;
> - if (pv_eoi_get_user(vcpu, &val) < 0)
> - return false;
> -
> - return val & KVM_PV_EOI_ENABLED;
> -}
> -
> static void pv_eoi_set_pending(struct kvm_vcpu *vcpu)
> {
> if (pv_eoi_put_user(vcpu, KVM_PV_EOI_ENABLED) < 0)
> @@ -690,12 +681,25 @@ static void pv_eoi_set_pending(struct kvm_vcpu *vcpu)
> __set_bit(KVM_APIC_PV_EOI_PENDING, &vcpu->arch.apic_attention);
> }
>
> -static void pv_eoi_clr_pending(struct kvm_vcpu *vcpu)
> +static bool pv_eoi_test_and_clr_pending(struct kvm_vcpu *vcpu)
> {
> - if (pv_eoi_put_user(vcpu, KVM_PV_EOI_DISABLED) < 0)
> - return;
> + u8 val;
> +
> + if (pv_eoi_get_user(vcpu, &val) < 0)
> + return false;
> +
> + val &= KVM_PV_EOI_ENABLED;
>
> + /*
> + * Clear pending bit in any case: it will be set again on vmentry.
> + * While this might not be ideal from performance point of view,
> + * this makes sure pv eoi is only enabled when we know it's safe.
> + */
This comment is misplaced now, as one may read it as we're clearing the
bit in guest's memory while in fact it refers to a bit in
'vcpu->arch.apic_attention'. Moreover, we're now checking 'val' so 'in
any case' above is certainly misleading.
We can add a
"Disable PV EOI in guest's memory in case it was previously
enabled" (or something like that)
comment here (but I don't think it gives much value to be honest).
> + if (val && pv_eoi_put_user(vcpu, KVM_PV_EOI_DISABLED) < 0)
> + return false;
Let's move the original comment here.
> __clear_bit(KVM_APIC_PV_EOI_PENDING, &vcpu->arch.apic_attention);
> +
> + return !!val;
('!!' is not really needed, the function returns bool)
> }
>
> static int apic_has_interrupt_for_ppr(struct kvm_lapic *apic, u32 ppr)
> @@ -2671,7 +2675,6 @@ void __kvm_migrate_apic_timer(struct kvm_vcpu *vcpu)
> static void apic_sync_pv_eoi_from_guest(struct kvm_vcpu *vcpu,
> struct kvm_lapic *apic)
> {
> - bool pending;
> int vector;
> /*
> * PV EOI state is derived from KVM_APIC_PV_EOI_PENDING in host
> @@ -2685,14 +2688,8 @@ static void apic_sync_pv_eoi_from_guest(struct kvm_vcpu *vcpu,
> * -> host enabled PV EOI, guest executed EOI.
> */
> BUG_ON(!pv_eoi_enabled(vcpu));
> - pending = pv_eoi_get_pending(vcpu);
> - /*
> - * Clear pending bit in any case: it will be set again on vmentry.
> - * While this might not be ideal from performance point of view,
> - * this makes sure pv eoi is only enabled when we know it's safe.
> - */
> - pv_eoi_clr_pending(vcpu);
> - if (pending)
> +
> + if (pv_eoi_test_and_clr_pending(vcpu))
> return;
> vector = apic_set_eoi(apic);
> trace_kvm_pv_eoi(apic, vector);
--
Vitaly
