linux-kernel - [PATCH v3 5/5] KVM: LAPIC: Narrow the timer latency between wait_lapic

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Date:   Mon, 17 May 2021 07:00:28 -0700
From:   Wanpeng Li <kernellwp@...il.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: [PATCH v3 5/5] KVM: LAPIC: Narrow the timer latency between wait_lapic_expire and world switch

From: Wanpeng Li <wanpengli@...cent.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@...cent.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