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KVM: LAPIC: Narrow the timer latency between wait_lapic_expire and world switch

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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 <wanpengli@tencent.com>
Message-Id: <1621339235-11131-5-git-send-email-wanpengli@tencent.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Wanpeng Li 2021-05-18 05:00:35 -07:00 committed by Paolo Bonzini
parent fb0f94794b
commit 9805cf03fd
1 changed files with 11 additions and 3 deletions
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14
arch/x86/kvm/lapic.c
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@ -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)