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[213.175.37.10]) by smtp.gmail.com with ESMTPSA id c14sm11510623wru.24.2019.10.22.06.11.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 22 Oct 2019 06:11:21 -0700 (PDT) From: Vitaly Kuznetsov To: Zhenzhong Duan , linux-kernel@vger.kernel.org Cc: tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org, pbonzini@redhat.com, rkrcmar@redhat.com, sean.j.christopherson@intel.com, wanpengli@tencent.com, jmattson@google.com, joro@8bytes.org, boris.ostrovsky@oracle.com, jgross@suse.com, peterz@infradead.org, will@kernel.org, linux-hyperv@vger.kernel.org, kvm@vger.kernel.org, mikelley@microsoft.com, kys@microsoft.com, haiyangz@microsoft.com, sthemmin@microsoft.com, sashal@kernel.org, Jonathan Corbet , "H. Peter Anvin" Subject: Re: [PATCH v7 3/5] x86/kvm: Add "nopvspin" parameter to disable PV spinlocks In-Reply-To: References: <1571649076-2421-1-git-send-email-zhenzhong.duan@oracle.com> <1571649076-2421-4-git-send-email-zhenzhong.duan@oracle.com> <8736fl1071.fsf@vitty.brq.redhat.com> Date: Tue, 22 Oct 2019 15:11:19 +0200 Message-ID: <87tv81ylfs.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org Zhenzhong Duan writes: > Hi Vitaly, > > On 2019/10/22 19:36, Vitaly Kuznetsov wrote: > >> Zhenzhong Duan writes: >> > ...snip > >>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c >>> index 249f14a..3945aa5 100644 >>> --- a/arch/x86/kernel/kvm.c >>> +++ b/arch/x86/kernel/kvm.c >>> @@ -825,18 +825,36 @@ __visible bool __kvm_vcpu_is_preempted(long cpu) >>> */ >>> void __init kvm_spinlock_init(void) >>> { >>> - /* Does host kernel support KVM_FEATURE_PV_UNHALT? */ >>> - if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)) >>> + /* >>> + * In case host doesn't support KVM_FEATURE_PV_UNHALT there is still an >>> + * advantage of keeping virt_spin_lock_key enabled: virt_spin_lock() is >>> + * preferred over native qspinlock when vCPU is preempted. >>> + */ >>> + if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)) { >>> + pr_info("PV spinlocks disabled, no host support.\n"); >>> return; >>> + } >>> >>> + /* >>> + * Disable PV qspinlock and use native qspinlock when dedicated pCPUs >>> + * are available. >>> + */ >>> if (kvm_para_has_hint(KVM_HINTS_REALTIME)) { >>> - static_branch_disable(&virt_spin_lock_key); >>> - return; >>> + pr_info("PV spinlocks disabled with KVM_HINTS_REALTIME hints.\n"); >>> + goto out; >>> } >>> >>> - /* Don't use the pvqspinlock code if there is only 1 vCPU. */ >>> - if (num_possible_cpus() == 1) >>> - return; >>> + if (num_possible_cpus() == 1) { >>> + pr_info("PV spinlocks disabled, single CPU.\n"); >>> + goto out; >>> + } >>> + >>> + if (nopvspin) { >>> + pr_info("PV spinlocks disabled, forced by \"nopvspin\" parameter.\n"); >>> + goto out; >>> + } >>> + >>> + pr_info("PV spinlocks enabled\n"); >>> >>> __pv_init_lock_hash(); >>> pv_ops.lock.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath; >>> @@ -849,6 +867,8 @@ void __init kvm_spinlock_init(void) >>> pv_ops.lock.vcpu_is_preempted = >>> PV_CALLEE_SAVE(__kvm_vcpu_is_preempted); >>> } >>> +out: >>> + static_branch_disable(&virt_spin_lock_key); >> You probably need to add 'return' before 'out:' as it seems you're >> disabling virt_spin_lock_key in all cases now). > > virt_spin_lock_key is kept enabled in !kvm_para_has_feature(KVM_FEATURE_PV_UNHALT) > case which is the only case virt_spin_lock() optimization is used. > > When PV qspinlock is enabled, virt_spin_lock() isn't called in > __pv_queued_spin_lock_slowpath() in which case we don't care > virt_spin_lock_key's value. > True, my bad: I though we still need it enabled for something. > So adding 'return' or not are both ok, I chosed to save a line, > let me know if you prefer to add a 'return' and I'll change it. No, please ignore. > > btw: __pv_queued_spin_lock_slowpath() is alias of queued_spin_lock_slowpath() > > Thanks > Zhenzhong > -- Vitaly