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[54.240.197.234]) by smtp.gmail.com with ESMTPSA id v4-20020a05600c214400b003fef19bb55csm4078475wml.34.2023.09.22.02.22.57 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 22 Sep 2023 02:22:58 -0700 (PDT) From: Paul Durrant X-Google-Original-From: Paul Durrant Message-ID: Date: Fri, 22 Sep 2023 10:22:56 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Reply-To: paul@xen.org Subject: Re: [PATCH] KVM: x86: Use fast path for Xen timer delivery To: David Woodhouse , kvm Cc: Sean Christopherson , Paolo Bonzini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , linux-kernel@vger.kernel.org, "Griffoul, Fred" References: <445abd6377cf1621a2d306226cab427820583967.camel@infradead.org> Content-Language: en-US Organization: Xen Project In-Reply-To: <445abd6377cf1621a2d306226cab427820583967.camel@infradead.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 22/09/2023 10:20, David Woodhouse wrote: > From: David Woodhouse > > Most of the time there's no need to kick the vCPU and deliver the timer > event through kvm_xen_inject_timer_irqs(). Use kvm_xen_set_evtchn_fast() > directly from the timer callback, and only fall back to the slow path > when it's necessary to do so. > > This gives a significant improvement in timer latency testing (using > nanosleep() for various periods and then measuring the actual time > elapsed). > > Signed-off-by: David Woodhouse > --- > arch/x86/kvm/xen.c | 17 ++++++++++++++--- > 1 file changed, 14 insertions(+), 3 deletions(-) > > diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c > index 40edf4d1974c..66c4cf93a55c 100644 > --- a/arch/x86/kvm/xen.c > +++ b/arch/x86/kvm/xen.c > @@ -134,12 +134,23 @@ static enum hrtimer_restart xen_timer_callback(struct hrtimer *timer) > { > struct kvm_vcpu *vcpu = container_of(timer, struct kvm_vcpu, > arch.xen.timer); > + struct kvm_xen_evtchn e; > + int rc; > + > if (atomic_read(&vcpu->arch.xen.timer_pending)) > return HRTIMER_NORESTART; > > - atomic_inc(&vcpu->arch.xen.timer_pending); > - kvm_make_request(KVM_REQ_UNBLOCK, vcpu); > - kvm_vcpu_kick(vcpu); > + e.vcpu_id = vcpu->vcpu_id; > + e.vcpu_idx = vcpu->vcpu_idx; > + e.port = vcpu->arch.xen.timer_virq; Don't you need to check the port for validity? The VMM may not have set it (and hence be delivering timer VIRQ events itself). Paul > + e.priority = KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL; > + > + rc = kvm_xen_set_evtchn_fast(&e, vcpu->kvm); > + if (rc == -EWOULDBLOCK) { > + atomic_inc(&vcpu->arch.xen.timer_pending); > + kvm_make_request(KVM_REQ_UNBLOCK, vcpu); > + kvm_vcpu_kick(vcpu); > + } > > return HRTIMER_NORESTART; > }