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Re: [PATCH v2 6/8] evtchn: convert vIRQ lock to an r/w one

Hi Jan,

On 20/10/2020 15:10, Jan Beulich wrote:
There's no need to serialize all sending of vIRQ-s; all that's needed
is serialization against the closing of the respective event channels
(so far by means of a barrier). To facilitate the conversion, switch to
an ordinary write locked region in evtchn_close().

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
v2: Don't introduce/use rw_barrier() here. Add comment to
     evtchn_bind_virq(). Re-base.

--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -160,7 +160,7 @@ struct vcpu *vcpu_create(struct domain *
      v->vcpu_id = vcpu_id;
      v->dirty_cpu = VCPU_CPU_CLEAN;
- spin_lock_init(&v->virq_lock);
+    rwlock_init(&v->virq_lock);
tasklet_init(&v->continue_hypercall_tasklet, NULL, NULL); --- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -449,6 +449,13 @@ int evtchn_bind_virq(evtchn_bind_virq_t
spin_unlock_irqrestore(&chn->lock, flags); + /*
+     * If by any, the update of virq_to_evtchn[] would need guarding by
+     * virq_lock, but since this is the last action here, there's no strict
+     * need to acquire the lock. Hnece holding event_lock isn't helpful


+     * anymore at this point, but utilize that its unlocking acts as the
+     * otherwise necessary smp_wmb() here.
+     */
      v->virq_to_evtchn[virq] = bind->port = port;

I think all access to v->virq_to_evtchn[virq] should use ACCESS_ONCE() or {read, write}_atomic() to avoid any store/load tearing.


Julien Grall



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