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From: Wanpeng Li <kernellwp@gmail.com>
To: linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>
Subject: [PATCH v3 5/5] KVM: LAPIC: Narrow the timer latency between wait_lapic_expire and world switch
Date: Mon, 17 May 2021 07:00:28 -0700	[thread overview]
Message-ID: <1621260028-6467-5-git-send-email-wanpengli@tencent.com> (raw)
In-Reply-To: <1621260028-6467-1-git-send-email-wanpengli@tencent.com>

From: Wanpeng Li <wanpengli@tencent.com>

Let's treat lapic_timer_advance_ns automatically tune logic as hypervisor
overhead, move it before wait_lapic_expire instead of between wait_lapic_expire 
and the world switch, the wait duration should be calculated by the 
up-to-date guest_tsc after the overhead of automatically tune logic. This 
patch reduces ~30+ cycles for kvm-unit-tests/tscdeadline-latency when testing 
busy waits.

Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 arch/x86/kvm/lapic.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index c0ebef560bd1..552d2acf89ab 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1598,11 +1598,12 @@ static void __kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
 	guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
 	apic->lapic_timer.advance_expire_delta = guest_tsc - tsc_deadline;
 
-	if (guest_tsc < tsc_deadline)
-		__wait_lapic_expire(vcpu, tsc_deadline - guest_tsc);
-
 	if (lapic_timer_advance_dynamic)
 		adjust_lapic_timer_advance(vcpu, apic->lapic_timer.advance_expire_delta);
+
+	guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
+	if (guest_tsc < tsc_deadline)
+		__wait_lapic_expire(vcpu, tsc_deadline - guest_tsc);
 }
 
 void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
-- 
2.25.1


  parent reply	other threads:[~2021-05-17 14:01 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-17 14:00 [PATCH v3 1/5] KVM: exit halt polling on need_resched() for both book3s and generic halt-polling Wanpeng Li
2021-05-17 14:00 ` [PATCH v3 2/5] KVM: X86: Bail out of direct yield in case of under-committed scenarios Wanpeng Li
2021-05-17 14:00 ` [PATCH v3 3/5] KVM: X86: Fix vCPU preempted state from guest's point of view Wanpeng Li
2021-05-17 14:00 ` [PATCH v3 4/5] KVM: x86: hyper-v: Task srcu lock when accessing kvm_memslots() Wanpeng Li
2021-05-17 14:00 ` Wanpeng Li [this message]
2021-05-17 17:51   ` [PATCH v3 5/5] KVM: LAPIC: Narrow the timer latency between wait_lapic_expire and world switch Sean Christopherson
2021-05-18 12:03     ` Wanpeng Li
2021-05-17 16:34 ` [PATCH v3 1/5] KVM: exit halt polling on need_resched() for both book3s and generic halt-polling David Matlack
2021-05-18 12:03   ` Wanpeng Li
2021-05-17 18:13 ` Venkatesh Srinivas

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