| Message ID | 20211009021236.4122790-29-seanjc@google.com (mailing list archive) |
|---|---|
| State | Not Applicable |
| Headers | show |
| Series | KVM: Halt-polling and x86 APICv overhaul | expand |
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 <seanjc@google.com> > --- > 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. > + * 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. > + * 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. > +/* > + * 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? > + > + 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); > + > vcpu->pre_pcpu = -1; > } > Best regards, Maxim Levitsky
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 <seanjc@google.com> > > --- > > 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.
On Thu, 2021-10-28 at 17:19 +0000, Sean Christopherson wrote: > 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 <seanjc@google.com> > > > --- > > > 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: Then no need for a helper. > > 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. > Makes sense! Best regards, Maxim Levitsky
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() + * 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. + * 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); +/* + * 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; + + 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); + vcpu->pre_pcpu = -1; }
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 <seanjc@google.com> --- arch/x86/kvm/vmx/posted_intr.c | 49 +++++++++++++++++++++++----------- 1 file changed, 33 insertions(+), 16 deletions(-)