In qvring_init() we're writing vq->used->avail_event at "vq->used + 2 +
array_size".  The struct pointed by vq->used is, from virtio_ring.h
Linux header):

 *	// A ring of used descriptor heads with free-running index.
 *	__virtio16 used_flags;
 *	__virtio16 used_idx;
 *	struct vring_used_elem used[num];
 *	__virtio16 avail_event_idx;

So 'flags' is the word right at vq->used. 'idx' is vq->used + 2. We need
to skip 'used_idx' by adding + 2 bytes, and then sum the vector size, to
reach avail_event_idx. An example on how to properly access this field
can be found in qvirtqueue_kick():

avail_event = qvirtio_readw(d, qts, vq->used + 4 +
                            sizeof(struct vring_used_elem) * vq->size);

This error was detected when enabling the RISC-V 'virt' libqos machine.
The 'idx' test from vhost-user-blk-test.c errors out with a timeout in
qvirtio_wait_used_elem(). The timeout happens because when processing
the first element, 'avail_event' is read in qvirtqueue_kick() as non-zero
because we didn't initialize it properly (and the memory at that point
happened to be non-zero). 'idx' is 0.

All of this makes this condition fail because "idx - avail_event" will
overflow and be non-zero:

/* < 1 because we add elements to avail queue one by one */
if ((flags & VRING_USED_F_NO_NOTIFY) == 0 &&
                        (!vq->event || (uint16_t)(idx-avail_event) < 1)) {
    d->bus->virtqueue_kick(d, vq);
}

As a result the virtqueue is never kicked and we'll timeout waiting for it.

Fixes: 1053587c3f ("libqos: Added EVENT_IDX support")
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
---
 tests/qtest/libqos/virtio.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/qtest/libqos/virtio.c b/tests/qtest/libqos/virtio.c
index 4f39124eba..82a6e122bf 100644
--- a/tests/qtest/libqos/virtio.c
+++ b/tests/qtest/libqos/virtio.c
@@ -265,7 +265,7 @@ void qvring_init(QTestState *qts, const QGuestAllocator *alloc, QVirtQueue *vq,
     /* vq->used->idx */
     qvirtio_writew(vq->vdev, qts, vq->used + 2, 0);
     /* vq->used->avail_event */
-    qvirtio_writew(vq->vdev, qts, vq->used + 2 +
+    qvirtio_writew(vq->vdev, qts, vq->used + 4 +
                    sizeof(struct vring_used_elem) * vq->size, 0);
 }
 
-- 
2.43.2