linux-kernel - Re: [PATCH v2 5/9] KVM: LAPIC: Narrow down the kick target vCPU

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Message-ID: <9281f28a-e2bd-a79c-fc94-4999f4c6307a@redhat.com>
Date:   Sat, 12 Sep 2020 08:46:15 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v2 5/9] KVM: LAPIC: Narrow down the kick target vCPU

On 10/09/20 11:50, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> The kick after setting KVM_REQ_PENDING_TIMER is used to handle the timer
> fires on a different pCPU which vCPU is running on, this kick is expensive
> since memory barrier, rcu, preemption disable/enable operations. We don't
> need this kick when injecting already-expired timer, we also should 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.
> 
> Reviewed-by: Sean Christopherson <sean.j.christopherson@...el.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/lapic.c | 4 +++-
>  arch/x86/kvm/x86.c   | 6 ------
>  arch/x86/kvm/x86.h   | 1 -
>  3 files changed, 3 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index e446bdf..3b32d3b 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1642,7 +1642,9 @@ 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_make_request(KVM_REQ_PENDING_TIMER, vcpu);
> +	if (from_timer_fn)
> +		kvm_vcpu_kick(vcpu);
>  }

This only matters for !APICv, but it's nice anyway.

Paolo

>  static void start_sw_tscdeadline(struct kvm_lapic *apic)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d39d6cf..dcf4494 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1774,12 +1774,6 @@ static s64 get_kvmclock_base_ns(void)
>  }
>  #endif
>  
> -void kvm_set_pending_timer(struct kvm_vcpu *vcpu)
> -{
> -	kvm_make_request(KVM_REQ_PENDING_TIMER, vcpu);
> -	kvm_vcpu_kick(vcpu);
> -}
> -
>  static void kvm_write_wall_clock(struct kvm *kvm, gpa_t wall_clock)
>  {
>  	int version;
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 995ab69..ea20b8b 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -246,7 +246,6 @@ 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_inject_realmode_interrupt(struct kvm_vcpu *vcpu, int irq, int inc_eip);
>  
>  void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr);
>