libqos/virtio.c: fix 'avail_event' offset in qvring_init()

libqos/virtio.c: fix 'avail_event' offset in qvring_init()

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>
Message-ID: <20240217192607.32565-3-dbarboza@ventanamicro.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>