"Michael S. Tsirkin" <m...@redhat.com> writes: > On Wed, Mar 11, 2015 at 04:29:32PM +1030, Rusty Russell wrote: >> We only fill in the 'req->qiov.size' bytes on a (successful) read, >> not on a write. >> >> Signed-off-by: Rusty Russell <ru...@rustcorp.com.au> >> --- >> hw/block/virtio-blk.c | 10 +++++++++- >> 1 file changed, 9 insertions(+), 1 deletion(-) >> >> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c >> index 258bb4c..98d87a9 100644 >> --- a/hw/block/virtio-blk.c >> +++ b/hw/block/virtio-blk.c >> @@ -50,11 +50,19 @@ static void virtio_blk_complete_request(VirtIOBlockReq >> *req, >> { >> VirtIOBlock *s = req->dev; >> VirtIODevice *vdev = VIRTIO_DEVICE(s); >> + int type = virtio_ldl_p(VIRTIO_DEVICE(req->dev), &req->out.type); >> >> trace_virtio_blk_req_complete(req, status); >> >> stb_p(&req->in->status, status); >> - virtqueue_push(s->vq, &req->elem, req->qiov.size + sizeof(*req->in)); >> + >> + /* If we didn't succeed, we *may* have written more, but don't >> + * count on it. */ > > I wonder about this. > So length as you specify it is <= actually written length. > What are the advantages of this approach? > How about we do the reverse, specify that the length in descriptor > is >= the size actually written? > > If we do this, all these buggy hosts suddenly become correct, > which seems better.
The point of telling the guest the amount written is that they don't have to zero the receive buffer beforehand. Cheers, Rusty.