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

[Stefan Hajnoczi <stefanha@redhat.com>]

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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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