On 25/03/2011 16:53, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:
> This lock is only intended to prevent racing with closing an event
> channel - evtchn_set_pending() doesn't require external serialization.
Still a small critical section, I doubt the serialisation matters compared
with the cost of an extra LOCKed operation on read_unlock(). Especially
since the lock is per-vcpu anyway and the only at-all common VIRQ is
VIRQ_TIMER. The probability of unnecessary serialisation being a bottleneck
here is basically zero.
We can get rid of the local_irq_save/restore operations though (at the cost
of changing the lock debug checks in spinlock.c because the send_*_virq
functions can be called both with IRQs disabled and enabled, so as-is the
patch could make us bug out on the checks). But, overall, worth it? I doubt
EFLAGS.IF fiddling compares unfavourably with an extra LOCKed operation,
cost wise.
So, I'm not even slightly convinced. I'll leave as is.
-- Keir
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>
>
> --- a/xen/common/domain.c
> +++ b/xen/common/domain.c
> @@ -147,7 +147,7 @@ struct vcpu *alloc_vcpu(
> v->domain = d;
> v->vcpu_id = vcpu_id;
>
> - spin_lock_init(&v->virq_lock);
> + rwlock_init(&v->virq_lock);
>
> tasklet_init(&v->continue_hypercall_tasklet, NULL, 0);
>
> --- a/xen/common/event_channel.c
> +++ b/xen/common/event_channel.c
> @@ -417,7 +417,7 @@ static long __evtchn_close(struct domain
> if ( v->virq_to_evtchn[chn1->u.virq] != port1 )
> continue;
> v->virq_to_evtchn[chn1->u.virq] = 0;
> - spin_barrier_irq(&v->virq_lock);
> + write_barrier_irq(&v->virq_lock);
> }
> break;
>
> @@ -613,12 +613,11 @@ int guest_enabled_event(struct vcpu *v,
>
> void send_guest_vcpu_virq(struct vcpu *v, int virq)
> {
> - unsigned long flags;
> int port;
>
> ASSERT(!virq_is_global(virq));
>
> - spin_lock_irqsave(&v->virq_lock, flags);
> + read_lock(&v->virq_lock);
>
> port = v->virq_to_evtchn[virq];
> if ( unlikely(port == 0) )
> @@ -627,12 +626,11 @@ void send_guest_vcpu_virq(struct vcpu *v
> evtchn_set_pending(v, port);
>
> out:
> - spin_unlock_irqrestore(&v->virq_lock, flags);
> + read_unlock(&v->virq_lock);
> }
>
> void send_guest_global_virq(struct domain *d, int virq)
> {
> - unsigned long flags;
> int port;
> struct vcpu *v;
> struct evtchn *chn;
> @@ -646,7 +644,7 @@ void send_guest_global_virq(struct domai
> if ( unlikely(v == NULL) )
> return;
>
> - spin_lock_irqsave(&v->virq_lock, flags);
> + read_lock(&v->virq_lock);
>
> port = v->virq_to_evtchn[virq];
> if ( unlikely(port == 0) )
> @@ -656,7 +654,7 @@ void send_guest_global_virq(struct domai
> evtchn_set_pending(d->vcpu[chn->notify_vcpu_id], port);
>
> out:
> - spin_unlock_irqrestore(&v->virq_lock, flags);
> + read_unlock(&v->virq_lock);
> }
>
> int send_guest_pirq(struct domain *d, int pirq)
> --- a/xen/common/spinlock.c
> +++ b/xen/common/spinlock.c
> @@ -344,6 +344,20 @@ int _rw_is_write_locked(rwlock_t *lock)
> return _raw_rw_is_write_locked(&lock->raw);
> }
>
> +void _write_barrier(rwlock_t *lock)
> +{
> + check_lock(&lock->debug);
> + do { mb(); } while ( _raw_rw_is_locked(&lock->raw) );
> +}
> +
> +void _write_barrier_irq(rwlock_t *lock)
> +{
> + unsigned long flags;
> + local_irq_save(flags);
> + _write_barrier(lock);
> + local_irq_restore(flags);
> +}
> +
> #ifdef LOCK_PROFILE
>
> struct lock_profile_anc {
> --- a/xen/include/xen/sched.h
> +++ b/xen/include/xen/sched.h
> @@ -157,9 +157,12 @@ struct vcpu
> unsigned long pause_flags;
> atomic_t pause_count;
>
> - /* IRQ-safe virq_lock protects against delivering VIRQ to stale evtchn.
> */
> + /*
> + * Writer-side-IRQ-safe virq_lock protects against delivering VIRQ
> + * to stale evtchn.
> + */
> u16 virq_to_evtchn[NR_VIRQS];
> - spinlock_t virq_lock;
> + rwlock_t virq_lock;
>
> /* Bitmask of CPUs on which this VCPU may run. */
> cpumask_t cpu_affinity;
> --- a/xen/include/xen/spinlock.h
> +++ b/xen/include/xen/spinlock.h
> @@ -170,6 +170,9 @@ void _write_unlock_irqrestore(rwlock_t *
> int _rw_is_locked(rwlock_t *lock);
> int _rw_is_write_locked(rwlock_t *lock);
>
> +void _write_barrier(rwlock_t *lock);
> +void _write_barrier_irq(rwlock_t *lock);
> +
> #define spin_lock(l) _spin_lock(l)
> #define spin_lock_irq(l) _spin_lock_irq(l)
> #define spin_lock_irqsave(l, f) ((f) = _spin_lock_irqsave(l))
> @@ -223,4 +226,7 @@ int _rw_is_write_locked(rwlock_t *lock);
> #define rw_is_locked(l) _rw_is_locked(l)
> #define rw_is_write_locked(l) _rw_is_write_locked(l)
>
> +#define write_barrier(l) _write_barrier(l)
> +#define write_barrier_irq(l) _write_barrier_irq(l)
> +
> #endif /* __SPINLOCK_H__ */
>
>
>
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