From ddf54503e2bbed01958cf5fb16ad6378971d2468 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Radim=20Kr=C4=8Dm=C3=A1=C5=99?= Date: Wed, 2 Mar 2016 22:56:41 +0100 Subject: [PATCH] KVM: i8254: use atomic_t instead of pit.inject_lock MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The lock was an overkill, the same can be done with atomics. A mb() was added in kvm_pit_ack_irq, to pair with implicit barrier between pit_timer_fn and pit_do_work. The mb() prevents a race that could happen if pending == 0 and irq_ack == 0: kvm_pit_ack_irq: | pit_timer_fn: p = atomic_read(&ps->pending); | | atomic_inc(&ps->pending); | queue_work(pit_do_work); | pit_do_work: | atomic_xchg(&ps->irq_ack, 0); | return; atomic_set(&ps->irq_ack, 1); | if (p == 0) return; | where the interrupt would not be delivered in this tick of pit_timer_fn. PIT would have eventually delivered the interrupt, but we sacrifice perofmance to make sure that interrupts are not needlessly delayed. sfence isn't enough: atomic_dec_if_positive does atomic_read first and x86 can reorder loads before stores. lfence isn't enough: store can pass lfence, turning it into a nop. A compiler barrier would be more than enough as CPU needs to stall for unbelievably long to use fences. This patch doesn't do anything in kvm_pit_reset_reinject, because any order of resets can race, but the result differs by at most one interrupt, which is ok, because it's the same result as if the reset happened at a slightly different time. (Original code didn't protect the reset path with a proper lock, so users have to be robust.) Signed-off-by: Radim Krčmář Signed-off-by: Paolo Bonzini --- arch/x86/kvm/i8254.c | 56 ++++++++++++++++++-------------------------- arch/x86/kvm/i8254.h | 3 +-- 2 files changed, 24 insertions(+), 35 deletions(-) diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c index bdbb3f076e72..0f5655c50e0c 100644 --- a/arch/x86/kvm/i8254.c +++ b/arch/x86/kvm/i8254.c @@ -237,11 +237,13 @@ static void kvm_pit_ack_irq(struct kvm_irq_ack_notifier *kian) struct kvm_kpit_state *ps = container_of(kian, struct kvm_kpit_state, irq_ack_notifier); - spin_lock(&ps->inject_lock); + atomic_set(&ps->irq_ack, 1); + /* irq_ack should be set before pending is read. Order accesses with + * inc(pending) in pit_timer_fn and xchg(irq_ack, 0) in pit_do_work. + */ + smp_mb(); if (atomic_dec_if_positive(&ps->pending) > 0 && ps->reinject) queue_kthread_work(&ps->pit->worker, &ps->pit->expired); - ps->irq_ack = 1; - spin_unlock(&ps->inject_lock); } void __kvm_migrate_pit_timer(struct kvm_vcpu *vcpu) @@ -272,36 +274,25 @@ static void pit_do_work(struct kthread_work *work) struct kvm_vcpu *vcpu; int i; struct kvm_kpit_state *ps = &pit->pit_state; - int inject = 0; - /* Try to inject pending interrupts when - * last one has been acked. + if (ps->reinject && !atomic_xchg(&ps->irq_ack, 0)) + return; + + kvm_set_irq(kvm, kvm->arch.vpit->irq_source_id, 0, 1, false); + kvm_set_irq(kvm, kvm->arch.vpit->irq_source_id, 0, 0, false); + + /* + * Provides NMI watchdog support via Virtual Wire mode. + * The route is: PIT -> LVT0 in NMI mode. + * + * Note: Our Virtual Wire implementation does not follow + * the MP specification. We propagate a PIT interrupt to all + * VCPUs and only when LVT0 is in NMI mode. The interrupt can + * also be simultaneously delivered through PIC and IOAPIC. */ - spin_lock(&ps->inject_lock); - if (!ps->reinject) - inject = 1; - else if (ps->irq_ack) { - ps->irq_ack = 0; - inject = 1; - } - spin_unlock(&ps->inject_lock); - if (inject) { - kvm_set_irq(kvm, kvm->arch.vpit->irq_source_id, 0, 1, false); - kvm_set_irq(kvm, kvm->arch.vpit->irq_source_id, 0, 0, false); - - /* - * Provides NMI watchdog support via Virtual Wire mode. - * The route is: PIT -> PIC -> LVT0 in NMI mode. - * - * Note: Our Virtual Wire implementation is simplified, only - * propagating PIT interrupts to all VCPUs when they have set - * LVT0 to NMI delivery. Other PIC interrupts are just sent to - * VCPU0, and only if its LVT0 is in EXTINT mode. - */ - if (atomic_read(&kvm->arch.vapics_in_nmi_mode) > 0) - kvm_for_each_vcpu(i, vcpu, kvm) - kvm_apic_nmi_wd_deliver(vcpu); - } + if (atomic_read(&kvm->arch.vapics_in_nmi_mode) > 0) + kvm_for_each_vcpu(i, vcpu, kvm) + kvm_apic_nmi_wd_deliver(vcpu); } static enum hrtimer_restart pit_timer_fn(struct hrtimer *data) @@ -324,7 +315,7 @@ static enum hrtimer_restart pit_timer_fn(struct hrtimer *data) static inline void kvm_pit_reset_reinject(struct kvm_pit *pit) { atomic_set(&pit->pit_state.pending, 0); - pit->pit_state.irq_ack = 1; + atomic_set(&pit->pit_state.irq_ack, 1); } static void create_pit_timer(struct kvm *kvm, u32 val, int is_period) @@ -691,7 +682,6 @@ struct kvm_pit *kvm_create_pit(struct kvm *kvm, u32 flags) mutex_init(&pit->pit_state.lock); mutex_lock(&pit->pit_state.lock); - spin_lock_init(&pit->pit_state.inject_lock); pid = get_pid(task_tgid(current)); pid_nr = pid_vnr(pid); diff --git a/arch/x86/kvm/i8254.h b/arch/x86/kvm/i8254.h index c84990b42b5b..f8cf4b84f435 100644 --- a/arch/x86/kvm/i8254.h +++ b/arch/x86/kvm/i8254.h @@ -33,8 +33,7 @@ struct kvm_kpit_state { u32 speaker_data_on; struct mutex lock; struct kvm_pit *pit; - spinlock_t inject_lock; - unsigned long irq_ack; + atomic_t irq_ack; struct kvm_irq_ack_notifier irq_ack_notifier; };