untrusted comment: signature from openbsd 5.5 base secret key
RWRGy8gxk9N93xob/rIni4484cw6hOLhQpR5MP30JzQo6ETZ6S1po5tqNYDOLThyCBGwWQxEPKEG8fQdkD9JfD7KHwbDq8J7uQk=

OpenBSD 5.5 errata 18, Dec 10, 2014:

Several bugs in virtio(4) can lead to hangs with virtio devices,
like vio(4) and vioblk(4).

Apply patch using:
    
    signify -Vep /etc/signify/openbsd-55-base.pub -x 018_virtio.patch.sig -m - | \
        (cd /usr/src && patch -p0)

Then build and install a new kernel:

    cd /usr/src/sys/arch/`machine`/conf
    KK=`sysctl -n kern.osversion | cut -d# -f1`
    config $KK
    cd ../compile/$KK
    make
    make install

Index: sys/dev/pci/virtio.c
===================================================================
RCS file: /cvs/src/sys/dev/pci/virtio.c,v
retrieving revision 1.5
retrieving revision 1.5.6.2
diff -u -p -r1.5 -r1.5.6.2
--- sys/dev/pci/virtio.c	10 Mar 2013 21:58:02 -0000	1.5
+++ sys/dev/pci/virtio.c	9 Dec 2014 13:03:54 -0000	1.5.6.2
@@ -277,7 +277,6 @@ virtio_init_vq(struct virtio_softc *sc, 
 
 	/* enqueue/dequeue status */
 	vq->vq_avail_idx = 0;
-	vq->vq_avail_signalled = 0xffff;
 	vq->vq_used_idx = 0;
 	vq_sync_aring(sc, vq, BUS_DMASYNC_PREWRITE);
 	vq_sync_uring(sc, vq, BUS_DMASYNC_PREREAD);
@@ -658,6 +657,8 @@ publish_avail_idx(struct virtio_softc *s
 {
 	vq_sync_aring(sc, vq, BUS_DMASYNC_PREWRITE);
 	vq_sync_uring(sc, vq, BUS_DMASYNC_PREREAD);
+
+	virtio_membar_producer();
 	vq->vq_avail->idx = vq->vq_avail_idx;
 	vq_sync_aring(sc, vq, BUS_DMASYNC_POSTWRITE);
 	vq->vq_queued = 1;
@@ -684,17 +685,19 @@ virtio_enqueue_commit(struct virtio_soft
 notify:
 	if (notifynow) {
 		if (vq->vq_owner->sc_features & VIRTIO_F_RING_EVENT_IDX) {
-			uint16_t o = vq->vq_avail_signalled;
+			uint16_t o = vq->vq_avail->idx;
 			uint16_t n = vq->vq_avail_idx;
-			uint16_t t = VQ_AVAIL_EVENT(vq) + 1;
+			uint16_t t;
 			publish_avail_idx(sc, vq);
-			if ((o < n && o < t && t <= n)
-			    || (o > n && (o < t || t <= n))) {
+
+			virtio_membar_sync();
+			t = VQ_AVAIL_EVENT(vq) + 1;
+			if ((uint16_t)(n - t) < (uint16_t)(n - o))
 				sc->sc_ops->kick(sc, vq->vq_index);
-				vq->vq_avail_signalled = n;
-			}
 		} else {
 			publish_avail_idx(sc, vq);
+
+			virtio_membar_sync();
 			if (!(vq->vq_used->flags & VRING_USED_F_NO_NOTIFY))
 				sc->sc_ops->kick(sc, vq->vq_index);
 		}
@@ -747,6 +750,8 @@ virtio_dequeue(struct virtio_softc *sc, 
 		return ENOENT;
 	usedidx = vq->vq_used_idx++;
 	usedidx &= vq->vq_mask;
+
+	virtio_membar_consumer();
 	slot = vq->vq_used->ring[usedidx].id;
 	qe = &vq->vq_entries[slot];
 
@@ -797,6 +802,7 @@ virtio_postpone_intr(struct virtqueue *v
 
 	/* set the new event index: avail_ring->used_event = idx */
 	VQ_USED_EVENT(vq) = idx;
+	virtio_membar_sync();
 
 	vq_sync_aring(vq->vq_owner, vq, BUS_DMASYNC_PREWRITE);
 	vq->vq_queued++;
@@ -870,6 +876,8 @@ virtio_start_vq_intr(struct virtio_softc
 		VQ_USED_EVENT(vq) = vq->vq_used_idx;
 	else
 		vq->vq_avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
+
+	virtio_membar_sync();
 
 	vq_sync_aring(sc, vq, BUS_DMASYNC_PREWRITE);
 	vq->vq_queued++;
Index: sys/arch/i386/include/atomic.h
===================================================================
RCS file: /cvs/src/sys/arch/i386/include/atomic.h,v
retrieving revision 1.11
retrieving revision 1.11.8.1
diff -u -p -r1.11 -r1.11.8.1
--- sys/arch/i386/include/atomic.h	19 Nov 2012 15:18:06 -0000	1.11
+++ sys/arch/i386/include/atomic.h	9 Dec 2014 13:03:17 -0000	1.11.8.1
@@ -49,6 +49,13 @@
 #else
 #define LOCK
 #endif
+
+#define __membar(_f) do { __asm __volatile(_f ::: "memory"); } while (0)
+
+/* virtio needs MP membars even on SP kernels */
+#define virtio_membar_producer()	__membar("")
+#define virtio_membar_consumer()	__membar("")
+#define virtio_membar_sync()		__membar("lock; addl $0,0(%%esp)")
 
 static __inline u_int64_t
 i386_atomic_testset_uq(volatile u_int64_t *ptr, u_int64_t val)
Index: sys/arch/amd64/include/atomic.h
===================================================================
RCS file: /cvs/src/sys/arch/amd64/include/atomic.h,v
retrieving revision 1.10
retrieving revision 1.10.4.1
diff -u -p -r1.10 -r1.10.4.1
--- sys/arch/amd64/include/atomic.h	17 Feb 2014 10:01:32 -0000	1.10
+++ sys/arch/amd64/include/atomic.h	9 Dec 2014 13:03:17 -0000	1.10.4.1
@@ -54,6 +54,13 @@
 #else
 #define LOCK
 #endif
+
+#define __membar(_f) do { __asm __volatile(_f ::: "memory"); } while (0)
+
+/* virtio needs MP membars even on SP kernels */
+#define virtio_membar_producer()	__membar("")
+#define virtio_membar_consumer()	__membar("")
+#define virtio_membar_sync()		__membar("mfence")
 
 static __inline u_int64_t
 x86_atomic_testset_u64(volatile u_int64_t *ptr, u_int64_t val)