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KVM: i8254: use atomic_t instead of pit.inject_lock

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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>
This commit is contained in:
Radim Krčmář 2016-03-02 22:56:41 +01:00 committed by Paolo Bonzini
parent fd700a00dc
commit ddf54503e2
2 changed files with 24 additions and 35 deletions
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56
arch/x86/kvm/i8254.c
View File

@ -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);

3
arch/x86/kvm/i8254.h
View File

@ -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;
};