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[35.185.214.157]) by smtp.gmail.com with ESMTPSA id y19sm4044516pfn.23.2021.10.28.10.06.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 28 Oct 2021 10:06:47 -0700 (PDT) Date: Thu, 28 Oct 2021 17:06:43 +0000 From: Sean Christopherson To: Maxim Levitsky Cc: Marc Zyngier , Huacai Chen , Aleksandar Markovic , Paul Mackerras , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Christian Borntraeger , Janosch Frank , Paolo Bonzini , James Morse , Alexandru Elisei , Suzuki K Poulose , Atish Patra , David Hildenbrand , Cornelia Huck , Claudio Imbrenda , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-mips@vger.kernel.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, David Matlack , Oliver Upton , Jing Zhang Subject: Re: [PATCH v2 35/43] KVM: SVM: Signal AVIC doorbell iff vCPU is in guest mode Message-ID: References: <20211009021236.4122790-1-seanjc@google.com> <20211009021236.4122790-36-seanjc@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-mips@vger.kernel.org On Thu, Oct 28, 2021, Maxim Levitsky wrote: > On Fri, 2021-10-08 at 19:12 -0700, Sean Christopherson wrote: > > Signal the AVIC doorbell iff the vCPU is running in the guest. If the vCPU > > is not IN_GUEST_MODE, it's guaranteed to pick up any pending IRQs on the > > next VMRUN, which unconditionally processes the vIRR. > > > > Add comments to document the logic. > > > > Signed-off-by: Sean Christopherson > > --- > > arch/x86/kvm/svm/avic.c | 14 ++++++++++++-- > > 1 file changed, 12 insertions(+), 2 deletions(-) > > > > diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c > > index 208c5c71e827..cbf02e7e20d0 100644 > > --- a/arch/x86/kvm/svm/avic.c > > +++ b/arch/x86/kvm/svm/avic.c > > @@ -674,7 +674,12 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec) > > kvm_lapic_set_irr(vec, vcpu->arch.apic); > > smp_mb__after_atomic(); > > > > - if (avic_vcpu_is_running(vcpu)) { > > + /* > > + * Signal the doorbell to tell hardware to inject the IRQ if the vCPU > > + * is in the guest. If the vCPU is not in the guest, hardware will > > + * automatically process AVIC interrupts at VMRUN. > > + */ > > + if (vcpu->mode == IN_GUEST_MODE) { > > int cpu = READ_ONCE(vcpu->cpu); > > > > /* > > @@ -687,8 +692,13 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec) > > if (cpu != get_cpu()) > > wrmsrl(SVM_AVIC_DOORBELL, kvm_cpu_get_apicid(cpu)); > > put_cpu(); > > - } else > > + } else { > > + /* > > + * Wake the vCPU if it was blocking. KVM will then detect the > > + * pending IRQ when checking if the vCPU has a wake event. > > + */ > > kvm_vcpu_wake_up(vcpu); > > + } > > > > return 0; > > } > > It makes sense indeed to avoid ringing the doorbell when the vCPU is not in > the guest mode. > > I do wonder if we want to call kvm_vcpu_wake_up always otherwise, as the vCPU > might be just outside of the guest mode and not scheduled out. I don't know > how expensive is kvm_vcpu_wake_up in this case. IIUC, you're asking if we should do something like: if (vcpu->mode == IN_GUEST_MODE) { } else if (!is_vcpu_loaded(vcpu)) { kvm_vcpu_wake_up(); } The answer is that kvm_vcpu_wake_up(), which is effectively rcuwait_wake_up(), is very cheap except for specific configurations that may or may not be valid for production[*]. Practically speaking, is_vcpu_loaded() doesn't exist and should never exist because it's inherently racy. The closest we have would be else if (vcpu != kvm_get_running_vcpu()) { kvm_vcpu_wake_up(); } but that's extremely unlikely to be a net win because getting the current vCPU requires atomics to disable/re-enable preemption, especially if rcuwait_wake_up() is modified to avoid the rcu lock/unlock. TL;DR: rcuwait_wake_up() is cheap, and if it's too expensive, a better optimization would be to make it less expensive. [*] https://lkml.kernel.org/r/20211020110638.797389-1-pbonzini@redhat.com > Before this patch, the avic_vcpu_is_running would only be false when the vCPU > is scheduled out (e.g when vcpu_put was done on it) > > Best regards, > Maxim Levitsky >