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[35.247.111.240]) by smtp.gmail.com with ESMTPSA id k1sm4452219pfa.30.2021.05.17.10.51.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 17 May 2021 10:51:20 -0700 (PDT) Date: Mon, 17 May 2021 17:51:16 +0000 From: Sean Christopherson To: Wanpeng Li Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel Subject: Re: [PATCH v3 5/5] KVM: LAPIC: Narrow the timer latency between wait_lapic_expire and world switch Message-ID: References: <1621260028-6467-1-git-send-email-wanpengli@tencent.com> <1621260028-6467-5-git-send-email-wanpengli@tencent.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1621260028-6467-5-git-send-email-wanpengli@tencent.com> Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Mon, May 17, 2021, Wanpeng Li wrote: > From: Wanpeng Li > > 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 > --- > 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()); This is redundant and unnecessary if automatic tuning is disabled, or if the timer did not arrive early. A comment would also be helpful. E.g. I think this would micro-optimize all paths: diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c index c0ebef560bd1..5d91f2367c31 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) > + 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 >