2020-08-22 04:02:20

by Sean Christopherson

Subject: Re: [PATCH] KVM: LAPIC: Don't kick vCPU which is injecting already-expired timer

On Fri, Aug 21, 2020 at 05:17:34PM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <[email protected]>
>
> The kick after setting KVM_REQ_PENDING_TIMER is used to handle the timer
> fires on a different pCPU which vCPU is running on, we don't need this
> kick when injecting already-expired timer, this kick is expensive since
> memory barrier, rcu, preemption disable/enable operations. This patch
> reduces the overhead by don't kick vCPU which is injecting already-expired
> timer.

This should also call out the VMX preemption timer case, which also passes
from_timer_fn=false but doesn't need a kick because kvm_lapic_expired_hv_timer()
is called from the target vCPU.

> Signed-off-by: Wanpeng Li <[email protected]>
> ---
> arch/x86/kvm/lapic.c | 2 +-
> arch/x86/kvm/x86.c | 5 +++--
> arch/x86/kvm/x86.h | 2 +-
> 3 files changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 248095a..5b5ae66 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1642,7 +1642,7 @@ static void apic_timer_expired(struct kvm_lapic *apic, bool from_timer_fn)
> }
>
> atomic_inc(&apic->lapic_timer.pending);
> - kvm_set_pending_timer(vcpu);
> + kvm_set_pending_timer(vcpu, from_timer_fn);

My vote would be to open code kvm_set_pending_timer() here and drop the
helper, i.e.

kvm_make_request(KVM_REQ_PENDING_TIMER, vcpu);
if (from_timer_fn)
kvm_vcpu_kick(vcpu);

with that and an updated changelog:

Reviewed-by: Sean Christopherson <[email protected]>

> }
>
> static void start_sw_tscdeadline(struct kvm_lapic *apic)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 599d732..2a45405 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1778,10 +1778,11 @@ static s64 get_kvmclock_base_ns(void)
> }
> #endif
>
> -void kvm_set_pending_timer(struct kvm_vcpu *vcpu)
> +void kvm_set_pending_timer(struct kvm_vcpu *vcpu, bool should_kick)
> {
> kvm_make_request(KVM_REQ_PENDING_TIMER, vcpu);
> - kvm_vcpu_kick(vcpu);
> + if (should_kick)
> + kvm_vcpu_kick(vcpu);
> }
>
> static void kvm_write_wall_clock(struct kvm *kvm, gpa_t wall_clock)
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 995ab69..0eaae9c 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -246,7 +246,7 @@ static inline bool kvm_vcpu_latch_init(struct kvm_vcpu *vcpu)
> return is_smm(vcpu) || kvm_x86_ops.apic_init_signal_blocked(vcpu);
> }
>
> -void kvm_set_pending_timer(struct kvm_vcpu *vcpu);
> +void kvm_set_pending_timer(struct kvm_vcpu *vcpu, bool should_kick);
> void kvm_inject_realmode_interrupt(struct kvm_vcpu *vcpu, int irq, int inc_eip);
>
> void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr);
> --
> 2.7.4
>

2020-08-24 00:21:37

by Wanpeng Li

Subject: Re: [PATCH] KVM: LAPIC: Don't kick vCPU which is injecting already-expired timer

On Sat, 22 Aug 2020 at 12:01, Sean Christopherson
<[email protected]> wrote:
>
> On Fri, Aug 21, 2020 at 05:17:34PM +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <[email protected]>
> >
> > The kick after setting KVM_REQ_PENDING_TIMER is used to handle the timer
> > fires on a different pCPU which vCPU is running on, we don't need this
> > kick when injecting already-expired timer, this kick is expensive since
> > memory barrier, rcu, preemption disable/enable operations. This patch
> > reduces the overhead by don't kick vCPU which is injecting already-expired
> > timer.
>
> This should also call out the VMX preemption timer case, which also passes
> from_timer_fn=false but doesn't need a kick because kvm_lapic_expired_hv_timer()
> is called from the target vCPU.
>
> > Signed-off-by: Wanpeng Li <[email protected]>
> > ---
> > arch/x86/kvm/lapic.c | 2 +-
> > arch/x86/kvm/x86.c | 5 +++--
> > arch/x86/kvm/x86.h | 2 +-
> > 3 files changed, 5 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 248095a..5b5ae66 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -1642,7 +1642,7 @@ static void apic_timer_expired(struct kvm_lapic *apic, bool from_timer_fn)
> > }
> >
> > atomic_inc(&apic->lapic_timer.pending);
> > - kvm_set_pending_timer(vcpu);
> > + kvm_set_pending_timer(vcpu, from_timer_fn);
>
> My vote would be to open code kvm_set_pending_timer() here and drop the
> helper, i.e.
>
> kvm_make_request(KVM_REQ_PENDING_TIMER, vcpu);
> if (from_timer_fn)
> kvm_vcpu_kick(vcpu);
>
> with that and an updated changelog:

Agreed.

>
> Reviewed-by: Sean Christopherson <[email protected]>

Thanks.
Wanpeng