From 9805cf03fdb6828091fe09e4ef0fb544fca3eaf6 Mon Sep 17 00:00:00 2001 From: Wanpeng Li Date: Tue, 18 May 2021 05:00:35 -0700 Subject: [PATCH] KVM: LAPIC: Narrow the timer latency between wait_lapic_expire and world switch Let's treat lapic_timer_advance_ns automatic tuning 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 automatic tuning logic. This patch reduces ~30+ cycles for kvm-unit-tests/tscdeadline-latency when testing busy waits. Signed-off-by: Wanpeng Li Message-Id: <1621339235-11131-5-git-send-email-wanpengli@tencent.com> Reviewed-by: Sean Christopherson Signed-off-by: Paolo Bonzini --- arch/x86/kvm/lapic.c | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-) diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c index c0ebef5..5d91f23 100644 --- a/arch/x86/kvm/lapic.c +++ b/arch/x86/kvm/lapic.c @@ -1598,11 +1598,19 @@ 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 (lapic_timer_advance_dynamic) { + adjust_lapic_timer_advance(vcpu, apic->lapic_timer.advance_expire_delta); + /* + * If the timer fired early, reread the TSC to account for the + * overhead of the above adjustment to avoid waiting longer + * than is necessary. + */ + if (guest_tsc < tsc_deadline) + guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc()); + } + 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); } void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu) -- 2.7.4