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[35.185.214.157]) by smtp.gmail.com with ESMTPSA id d24sm3945465pfn.62.2021.10.28.10.19.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 28 Oct 2021 10:19:38 -0700 (PDT) Date: Thu, 28 Oct 2021 17:19:34 +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 28/43] KVM: VMX: Remove vCPU from PI wakeup list before updating PID.NV Message-ID: References: <20211009021236.4122790-1-seanjc@google.com> <20211009021236.4122790-29-seanjc@google.com> <558e7e4c36e649709837079a25c2f56fc5609fbe.camel@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <558e7e4c36e649709837079a25c2f56fc5609fbe.camel@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 28, 2021, Maxim Levitsky wrote: > On Fri, 2021-10-08 at 19:12 -0700, Sean Christopherson wrote: > > Remove the vCPU from the wakeup list before updating the notification > > vector in the posted interrupt post-block helper. There is no need to > > wake the current vCPU as it is by definition not blocking. Practically > > speaking this is a nop as it only shaves a few meager cycles in the > > unlikely case that the vCPU was migrated and the previous pCPU gets a > > wakeup IRQ right before PID.NV is updated. The real motivation is to > > allow for more readable code in the future, when post-block is merged > > with vmx_vcpu_pi_load(), at which point removal from the list will be > > conditional on the old notification vector. > > > > Opportunistically add comments to document why KVM has a per-CPU spinlock > > that, at first glance, appears to be taken only on the owning CPU. > > Explicitly call out that the spinlock must be taken with IRQs disabled, a > > detail that was "lost" when KVM switched from spin_lock_irqsave() to > > spin_lock(), with IRQs disabled for the entirety of the relevant path. > > > > Signed-off-by: Sean Christopherson > > --- > > arch/x86/kvm/vmx/posted_intr.c | 49 +++++++++++++++++++++++----------- > > 1 file changed, 33 insertions(+), 16 deletions(-) > > > > diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c > > index 2b2206339174..901b7a5f7777 100644 > > --- a/arch/x86/kvm/vmx/posted_intr.c > > +++ b/arch/x86/kvm/vmx/posted_intr.c > > @@ -10,10 +10,22 @@ > > #include "vmx.h" > > > > /* > > - * We maintain a per-CPU linked-list of vCPU, so in wakeup_handler() we > > - * can find which vCPU should be waken up. > > + * Maintain a per-CPU list of vCPUs that need to be awakened by wakeup_handler() > Nit: While at it, it would be nice to rename this to pi_wakeup_hanlder() so > that it can be more easilly found. Ah, good catch. > > + * when a WAKEUP_VECTOR interrupted is posted. vCPUs are added to the list when > > + * the vCPU is scheduled out and is blocking (e.g. in HLT) with IRQs enabled. > s/interrupted/interrupt ? > > Isn't that comment incorrect? As I see, the PI hardware is setup to use the WAKEUP_VECTOR > when vcpu blocks (in pi_pre_block) and then that vcpu is added to the list. > The pi_wakeup_hanlder just goes over the list and wakes up all vcpus on the lsit. Doh, yes. This patch is predicting the future. The comment becomes correct as of KVM: VMX: Handle PI wakeup shenanigans during vcpu_put/load but as of this patch the "scheduled out" piece doesn't hold true. > > + * The vCPUs posted interrupt descriptor is updated at the same time to set its > > + * notification vector to WAKEUP_VECTOR, so that posted interrupt from devices > > + * wake the target vCPUs. vCPUs are removed from the list and the notification > > + * vector is reset when the vCPU is scheduled in. > > */ > > static DEFINE_PER_CPU(struct list_head, blocked_vcpu_on_cpu); > Also while at it, why not to rename this to 'blocked_vcpu_list'? > to explain that this is list of blocked vcpus. Its a per-cpu variable > so 'on_cpu' suffix isn't needed IMHO. As you noted, addressed in a future patch. > > +/* > > + * Protect the per-CPU list with a per-CPU spinlock to handle task migration. > > + * When a blocking vCPU is awakened _and_ migrated to a different pCPU, the > > + * ->sched_in() path will need to take the vCPU off the list of the _previous_ > > + * CPU. IRQs must be disabled when taking this lock, otherwise deadlock will > > + * occur if a wakeup IRQ arrives and attempts to acquire the lock. > > + */ > > static DEFINE_PER_CPU(spinlock_t, blocked_vcpu_on_cpu_lock); > > > > static inline struct pi_desc *vcpu_to_pi_desc(struct kvm_vcpu *vcpu) > > @@ -101,23 +113,28 @@ static void __pi_post_block(struct kvm_vcpu *vcpu) > > WARN(pi_desc->nv != POSTED_INTR_WAKEUP_VECTOR, > > "Wakeup handler not enabled while the vCPU was blocking"); > > > > - dest = cpu_physical_id(vcpu->cpu); > > - if (!x2apic_mode) > > - dest = (dest << 8) & 0xFF00; > > - > > - do { > > - old.control = new.control = READ_ONCE(pi_desc->control); > > - > > - new.ndst = dest; > > - > > - /* set 'NV' to 'notification vector' */ > > - new.nv = POSTED_INTR_VECTOR; > > - } while (cmpxchg64(&pi_desc->control, old.control, > > - new.control) != old.control); > > - > > + /* > > + * Remove the vCPU from the wakeup list of the _previous_ pCPU, which > > + * will not be the same as the current pCPU if the task was migrated. > > + */ > > spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu)); > > list_del(&vcpu->blocked_vcpu_list); > > spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu)); > > + > > + dest = cpu_physical_id(vcpu->cpu); > > + if (!x2apic_mode) > > + dest = (dest << 8) & 0xFF00; > It would be nice to have a function for this, this appears in this file twice. > Maybe there is a function already somewhere? The second instance does go away by the aforementioned: KVM: VMX: Handle PI wakeup shenanigans during vcpu_put/load I'm inclined to say we don't want a helper because there should only ever be one path that changes PI.ndst. But a comment would definitely help to explain the difference between xAPIC and x2APIC IDs.