Re: [Qemu-devel] [PATCH] xen: Drop net_rx_ok

[Stefano Stabellini]

On Mon, 27 Jul 2015, Stefan Hajnoczi wrote:
> On Mon, Jul 20, 2015 at 06:12:09PM +0100, Stefan Hajnoczi wrote:
> > On Thu, Jul 02, 2015 at 01:39:16PM +0100, Stefan Hajnoczi wrote:
> > > On Tue, Jun 30, 2015 at 10:42:37AM +0800, Fam Zheng wrote:
> > > > This is necessary because once we return false from .can_receive, we
> > > > need to flush the queue when the .can_receive conditions become true
> > > > again, (for example when more buffer is available).
> > > > 
> > > > We can rely on net_rx_packet (which checks the same conditions) to drop
> > > > the packet if the device is not ready, so drop net_xen_info.can_receive.
> > > 
> > > This patch changes behavior:
> > > 
> > > Previously can_receive() false meant packets are queued.
> > > 
> > > Now those same conditions result in net_rx_packet() returning -1, so
> > > packets are discarded.
> > > 
> > > In order to keep the spirit of the queuing mechanism - where we tell a
> > > sender to hold off until more rx buffers become available - I think the
> > > following line in net_rx_packet() needs to be changed:
> > > 
> > >   if (rc == rp || RING_REQUEST_CONS_OVERFLOW(&netdev->rx_ring, rc)) {
> > >       xen_be_printf(&netdev->xendev, 2, "no buffer, drop packet\n");
> > >       return -1;  <-- this should be changed to return 0
> > >   }
> > > 
> > > That change assumes that net_event()'s flush is always called when the
> > > rx ring gets more free space.
> > > 
> > > Any thoughts from Xen folks?
> > 
> > Ping?
> 
> Need input from Xen developers.

FYI I don't have an opinion on this: I am not familiar with this code as
xen_nic is the one Xen PV backend in QEMU that is not used with Xen.
Theoretically it could be used, but actually there is no way to select
it from the toolstack.