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KVM: i8254: use atomic_t instead of pit.inject_lock
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 <rkrcmar@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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fd700a00dc
commit
ddf54503e2
2 changed files with 24 additions and 35 deletions
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@ -237,11 +237,13 @@ static void kvm_pit_ack_irq(struct kvm_irq_ack_notifier *kian)
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struct kvm_kpit_state *ps = container_of(kian, struct kvm_kpit_state,
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irq_ack_notifier);
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spin_lock(&ps->inject_lock);
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atomic_set(&ps->irq_ack, 1);
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/* irq_ack should be set before pending is read. Order accesses with
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* inc(pending) in pit_timer_fn and xchg(irq_ack, 0) in pit_do_work.
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*/
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smp_mb();
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if (atomic_dec_if_positive(&ps->pending) > 0 && ps->reinject)
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queue_kthread_work(&ps->pit->worker, &ps->pit->expired);
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ps->irq_ack = 1;
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spin_unlock(&ps->inject_lock);
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}
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void __kvm_migrate_pit_timer(struct kvm_vcpu *vcpu)
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@ -272,36 +274,25 @@ static void pit_do_work(struct kthread_work *work)
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struct kvm_vcpu *vcpu;
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int i;
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struct kvm_kpit_state *ps = &pit->pit_state;
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int inject = 0;
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/* Try to inject pending interrupts when
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* last one has been acked.
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if (ps->reinject && !atomic_xchg(&ps->irq_ack, 0))
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return;
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kvm_set_irq(kvm, kvm->arch.vpit->irq_source_id, 0, 1, false);
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kvm_set_irq(kvm, kvm->arch.vpit->irq_source_id, 0, 0, false);
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/*
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* Provides NMI watchdog support via Virtual Wire mode.
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* The route is: PIT -> LVT0 in NMI mode.
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*
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* Note: Our Virtual Wire implementation does not follow
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* the MP specification. We propagate a PIT interrupt to all
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* VCPUs and only when LVT0 is in NMI mode. The interrupt can
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* also be simultaneously delivered through PIC and IOAPIC.
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*/
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spin_lock(&ps->inject_lock);
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if (!ps->reinject)
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inject = 1;
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else if (ps->irq_ack) {
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ps->irq_ack = 0;
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inject = 1;
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}
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spin_unlock(&ps->inject_lock);
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if (inject) {
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kvm_set_irq(kvm, kvm->arch.vpit->irq_source_id, 0, 1, false);
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kvm_set_irq(kvm, kvm->arch.vpit->irq_source_id, 0, 0, false);
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/*
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* Provides NMI watchdog support via Virtual Wire mode.
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* The route is: PIT -> PIC -> LVT0 in NMI mode.
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*
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* Note: Our Virtual Wire implementation is simplified, only
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* propagating PIT interrupts to all VCPUs when they have set
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* LVT0 to NMI delivery. Other PIC interrupts are just sent to
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* VCPU0, and only if its LVT0 is in EXTINT mode.
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*/
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if (atomic_read(&kvm->arch.vapics_in_nmi_mode) > 0)
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kvm_for_each_vcpu(i, vcpu, kvm)
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kvm_apic_nmi_wd_deliver(vcpu);
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}
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if (atomic_read(&kvm->arch.vapics_in_nmi_mode) > 0)
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kvm_for_each_vcpu(i, vcpu, kvm)
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kvm_apic_nmi_wd_deliver(vcpu);
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}
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static enum hrtimer_restart pit_timer_fn(struct hrtimer *data)
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@ -324,7 +315,7 @@ static enum hrtimer_restart pit_timer_fn(struct hrtimer *data)
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static inline void kvm_pit_reset_reinject(struct kvm_pit *pit)
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{
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atomic_set(&pit->pit_state.pending, 0);
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pit->pit_state.irq_ack = 1;
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atomic_set(&pit->pit_state.irq_ack, 1);
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}
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static void create_pit_timer(struct kvm *kvm, u32 val, int is_period)
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@ -691,7 +682,6 @@ struct kvm_pit *kvm_create_pit(struct kvm *kvm, u32 flags)
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mutex_init(&pit->pit_state.lock);
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mutex_lock(&pit->pit_state.lock);
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spin_lock_init(&pit->pit_state.inject_lock);
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pid = get_pid(task_tgid(current));
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pid_nr = pid_vnr(pid);
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@ -33,8 +33,7 @@ struct kvm_kpit_state {
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u32 speaker_data_on;
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struct mutex lock;
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struct kvm_pit *pit;
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spinlock_t inject_lock;
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unsigned long irq_ack;
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atomic_t irq_ack;
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struct kvm_irq_ack_notifier irq_ack_notifier;
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};
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