[Qemu-devel] Re: [PATCH] e1000: clear EOP for multi-buffer descriptors

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

On Mon, Feb 07, 2011 at 02:30:26PM +0200, Michael S. Tsirkin wrote:
> The e1000 spec says: if software statically allocates
> buffers, and uses memory read to check for completed descriptors, it
> simply has to zero the status byte in the descriptor to make it ready
> for reuse by hardware. This is not a hardware requirement (moving the
> hardware tail pointer is), but is necessary for performing an in–memory
> scan.
> 
> Thus the guest does not have to clear the status byte.  In case it
> doesn't we need to clear EOP for all descriptors
> except the last.  While I don't know of any such guests,
> it's probably a good idea to stick to the spec.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Reported-by: Juan Quintela <quintela@redhat.com>
> 
> ---
>  hw/e1000.c |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)

This makes sense: if the guest didn't clear the end-of-packet bit but
we're receiving a multi-buffer packet, clear EOP for all but the last
descriptor.

Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>