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

[Stefan Hajnoczi]

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?

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