Re: [PATCH v3 2/5] evtchn: avoid access tearing for ->virq_to_evtchn[] accesses

[Julien Grall]

Hi Jan,

On 23/11/2020 13:28, Jan Beulich wrote:

Use {read,write}_atomic() to exclude any eventualities, in particular
observing that accesses aren't all happening under a consistent lock.

Requested-by: Julien Grall <jul...@xen.org>
Signed-off-by: Jan Beulich <jbeul...@suse.com>

Reviewed-by: Julien Grall <jgr...@amazon.com>

Cheers,

---
v3: New.

--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -446,7 +446,7 @@ int evtchn_bind_virq(evtchn_bind_virq_t

spin_lock(&d->event_lock); - if ( v->virq_to_evtchn[virq] != 0 )

+    if ( read_atomic(&v->virq_to_evtchn[virq]) )
          ERROR_EXIT(-EEXIST);

if ( port != 0 )

@@ -474,7 +474,8 @@ int evtchn_bind_virq(evtchn_bind_virq_t

evtchn_write_unlock(chn); - v->virq_to_evtchn[virq] = bind->port = port;

+    bind->port = port;
+    write_atomic(&v->virq_to_evtchn[virq], port);

out:

      spin_unlock(&d->event_lock);
@@ -660,9 +661,9 @@ int evtchn_close(struct domain *d1, int
      case ECS_VIRQ:
          for_each_vcpu ( d1, v )
          {
-            if ( v->virq_to_evtchn[chn1->u.virq] != port1 )
+            if ( read_atomic(&v->virq_to_evtchn[chn1->u.virq]) != port1 )
                  continue;
-            v->virq_to_evtchn[chn1->u.virq] = 0;
+            write_atomic(&v->virq_to_evtchn[chn1->u.virq], 0);
              spin_barrier(&v->virq_lock);
          }
          break;
@@ -801,7 +802,7 @@ bool evtchn_virq_enabled(const struct vc
      if ( virq_is_global(virq) && v->vcpu_id )
          v = domain_vcpu(v->domain, 0);

- return v->virq_to_evtchn[virq];

+    return read_atomic(&v->virq_to_evtchn[virq]);
  }

void send_guest_vcpu_virq(struct vcpu *v, uint32_t virq)

@@ -814,7 +815,7 @@ void send_guest_vcpu_virq(struct vcpu *v

spin_lock_irqsave(&v->virq_lock, flags); - port = v->virq_to_evtchn[virq];

+    port = read_atomic(&v->virq_to_evtchn[virq]);
      if ( unlikely(port == 0) )
          goto out;

@@ -843,7 +844,7 @@ void send_guest_global_virq(struct domai spin_lock_irqsave(&v->virq_lock, flags); - port = v->virq_to_evtchn[virq];

+    port = read_atomic(&v->virq_to_evtchn[virq]);
      if ( unlikely(port == 0) )
          goto out;

--
Julien Grall