[PATCH 50/58] xen/events: mask events when changing their VCPU binding

Kamal Mostafa kamal at canonical.com
Tue Sep 3 16:23:54 UTC 2013 -stable review patch.  If anyone has any objections, please let me know.


From: David Vrabel <david.vrabel at citrix.com>

commit 4704fe4f03a5ab27e3c36184af85d5000e0f8a48 upstream.

When a event is being bound to a VCPU there is a window between the
EVTCHNOP_bind_vpcu call and the adjustment of the local per-cpu masks
where an event may be lost.  The hypervisor upcalls the new VCPU but
the kernel thinks that event is still bound to the old VCPU and
ignores it.

There is even a problem when the event is being bound to the same VCPU
as there is a small window beween the clear_bit() and set_bit() calls
in bind_evtchn_to_cpu().  When scanning for pending events, the kernel
may read the bit when it is momentarily clear and ignore the event.

Avoid this by masking the event during the whole bind operation.

Signed-off-by: David Vrabel <david.vrabel at citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk at oracle.com>
Reviewed-by: Jan Beulich <jbeulich at suse.com>
[ kamal: backport to 3.8 ]
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
 drivers/xen/events.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/xen/events.c b/drivers/xen/events.c
index 6f788a8..4be58b3 100644
--- a/drivers/xen/events.c
+++ b/drivers/xen/events.c
@@ -1464,8 +1464,10 @@ void rebind_evtchn_irq(int evtchn, int irq)
 /* Rebind an evtchn so that it gets delivered to a specific cpu */
 static int rebind_irq_to_cpu(unsigned irq, unsigned tcpu)
+	struct shared_info *s = HYPERVISOR_shared_info;
 	struct evtchn_bind_vcpu bind_vcpu;
 	int evtchn = evtchn_from_irq(irq);
+	int masked;
 	if (!VALID_EVTCHN(evtchn))
 		return -1;
@@ -1482,6 +1484,12 @@ static int rebind_irq_to_cpu(unsigned irq, unsigned tcpu)
 	bind_vcpu.vcpu = tcpu;
+	 * Mask the event while changing the VCPU binding to prevent
+	 * it being delivered on an unexpected VCPU.
+	 */
+	masked = sync_test_and_set_bit(evtchn, s->evtchn_mask);
+	/*
 	 * If this fails, it usually just indicates that we're dealing with a
 	 * virq or IPI channel, which don't actually need to be rebound. Ignore
 	 * it, but don't do the xenlinux-level rebind in that case.
@@ -1489,6 +1497,9 @@ static int rebind_irq_to_cpu(unsigned irq, unsigned tcpu)
 	if (HYPERVISOR_event_channel_op(EVTCHNOP_bind_vcpu, &bind_vcpu) >= 0)
 		bind_evtchn_to_cpu(evtchn, tcpu);
+	if (!masked)
+		unmask_evtchn(evtchn);
 	return 0;

More information about the kernel-team mailing list