Hi Alex, On 4/6/22 7:33 PM, Alex Bennée wrote: > Eric noticed while attempting to enable the vhost-user-blk-test for > Aarch64 that that things didn't work unless he put in a dummy > guest_malloc() at the start of the test. Without it > qvirtio_wait_used_elem() would assert when it reads a junk value for > idx resulting in: > > qvirtqueue_get_buf: idx:2401 last_idx:0 > qvirtqueue_get_buf: 0x7ffcb6d3fe74, (nil) > qvirtio_wait_used_elem: 3000000/0 > ERROR:../../tests/qtest/libqos/virtio.c:226:qvirtio_wait_used_elem: > assertion failed (got_desc_idx == desc_idx): (50331648 == 0) > Bail out! > ERROR:../../tests/qtest/libqos/virtio.c:226:qvirtio_wait_used_elem: assertion > failed (got_desc_idx == desc_idx): (50331648 == 0) > > What was actually happening is the guest_malloc() effectively pushed > the allocation of the vring into the next page which just happened to > have clear memory. After much tedious tracing of the code I could see Many thanks for the tedious investigation! > that qvring_init() does attempt initialise a bunch of the vring > structures but skips the vring->used.idx value. It is probably not > wise to assume guest memory is zeroed anyway. Once the ring is > properly initialised the hack is no longer needed to get things > working. > > Thanks-to: John Snow <js...@redhat.com> for helping debug > Cc: Eric Auger <eric.au...@redhat.com> > Cc: Stefan Hajnoczi <stefa...@redhat.com> > Cc: Michael S. Tsirkin <m...@redhat.com> > Cc: Raphael Norwitz <raphael.norw...@nutanix.com> > Signed-off-by: Alex Bennée <alex.ben...@linaro.org> > --- > tests/qtest/libqos/virtio.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/tests/qtest/libqos/virtio.c b/tests/qtest/libqos/virtio.c > index 6fe7bf9555..fba9186659 100644 > --- a/tests/qtest/libqos/virtio.c > +++ b/tests/qtest/libqos/virtio.c > @@ -260,6 +260,8 @@ void qvring_init(QTestState *qts, const QGuestAllocator > *alloc, QVirtQueue *vq, > > /* vq->used->flags */ > qvirtio_writew(vq->vdev, qts, vq->used, 0); > + /* vq->used->idx */ > + qvirtio_writew(vq->vdev, qts, vq->used + 2, 0); > /* vq->used->avail_event */ > qvirtio_writew(vq->vdev, qts, vq->used + 2 + > sizeof(struct vring_used_elem) * vq->size, 0); Reviewed-by: Eric Auger <eric.au...@redhat.com> Tested-by: Eric Auger <eric.au...@redhat.com>
Eric
