From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:54172)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <rusty@ozlabs.org>) id 1YVexi-0001cd-Sh
	for qemu-devel@nongnu.org; Wed, 11 Mar 2015 07:38:07 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <rusty@ozlabs.org>) id 1YVexb-000481-PU
	for qemu-devel@nongnu.org; Wed, 11 Mar 2015 07:38:06 -0400
Received: from ozlabs.org ([2401:3900:2:1::2]:51331)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <rusty@ozlabs.org>) id 1YVexb-00047m-EI
	for qemu-devel@nongnu.org; Wed, 11 Mar 2015 07:37:59 -0400
From: Rusty Russell <rusty@rustcorp.com.au>
In-Reply-To: <20150311074415-mutt-send-email-mst@redhat.com>
References: <1426053572-21326-1-git-send-email-rusty@rustcorp.com.au>
	<1426053572-21326-3-git-send-email-rusty@rustcorp.com.au>
	<20150311074415-mutt-send-email-mst@redhat.com>
Date: Wed, 11 Mar 2015 22:04:02 +1030
Message-ID: <87sidbydhx.fsf@rustcorp.com.au>
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: [Qemu-devel] [PATCH 2/2] virtio-blk: fix length calculations
	for write operations.
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: QEMU Developers <qemu-devel@nongnu.org>

"Michael S. Tsirkin" <mst@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 <rusty@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.