Re: [Qemu-devel] [PATCH] virtio-net: Drop net_virtio_info.can_receive

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 02, 2015 at 01:46:26PM +0100, Stefan Hajnoczi wrote:
> On Tue, Jun 30, 2015 at 04:35:24PM +0800, Jason Wang wrote:
> > On 06/30/2015 11:06 AM, Fam Zheng wrote:
> > > virtio_net_receive still does the check by calling
> > > virtio_net_can_receive, if the device or driver is not ready, the packet
> > > is dropped.
> > >
> > > This is necessary because returning false from can_receive complicates
> > > things: the peer would disable sending until we explicitly flush the
> > > queue.
> > >
> > > Signed-off-by: Fam Zheng <famz@redhat.com>
> > > ---
> > >  hw/net/virtio-net.c | 1 -
> > >  1 file changed, 1 deletion(-)
> > >
> > > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > > index d728233..dbef0d0 100644
> > > --- a/hw/net/virtio-net.c
> > > +++ b/hw/net/virtio-net.c
> > > @@ -1503,7 +1503,6 @@ static int virtio_net_load_device(VirtIODevice *vdev, QEMUFile *f,
> > >  static NetClientInfo net_virtio_info = {
> > >      .type = NET_CLIENT_OPTIONS_KIND_NIC,
> > >      .size = sizeof(NICState),
> > > -    .can_receive = virtio_net_can_receive,
> > >      .receive = virtio_net_receive,
> > >      .link_status_changed = virtio_net_set_link_status,
> > >      .query_rx_filter = virtio_net_query_rxfilter,
> > 
> > A side effect of this patch is it will read and then drop packet is
> > guest driver is no ok.
> 
> I think that the semantics of .can_receive() and .receive() return
> values are currently incorrect in many NICs.  They have .can_receive()
> functions that return false for conditions where .receive() would
> discard the packet.  So what happens is that packets get queued when
> they should actually be discarded.
> 
> The purpose of the flow control (queuing) mechanism is to tell the
> sender to hold off until the receiver has more rx buffers available.
> It's a short-term thing that doesn't included link down, rx disable, or
> NIC reset states.
> 
> Therefore, I think this patch will not introduce a regression.  It is
> adjusting the code to stop queuing packets when they should actually be
> dropped.
> 
> Thoughts?
> 
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

OK, but we do have transient out of buffers states too, and
virtio_net_can_receive checks these as well.

To me, it looks like we should change virtio_net_can_receive to
avoid checking link down state, etc.
But I don't see why we should remove it completely.


-- 
MST