linux-kernel - [PATCH] KVM: x86: Use fast path for Xen timer delivery

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Message-ID: <445abd6377cf1621a2d306226cab427820583967.camel@infradead.org>
Date:   Fri, 22 Sep 2023 10:20:20 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     kvm <kvm@...r.kernel.org>
Cc:     Paul Durrant <paul@....org>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        "Griffoul, Fred" <fgriffo@...zon.com>
Subject: [PATCH] KVM: x86: Use fast path for Xen timer delivery

From: David Woodhouse <dwmw@...zon.co.uk>

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 <dwmw@...zon.co.uk>
---
 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;
+	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;
 }
-- 
2.34.1



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