Re: [Qemu-devel] [PATCH] e1000: Pad short frames to minimum size (60 bytes)

["Edgar E. Iglesias" <edgar.iglesias@gmail.com>]

On Sun, Sep 19, 2010 at 01:18:01PM +0200, Michael S. Tsirkin wrote:
> On Sun, Sep 19, 2010 at 07:36:51AM +0100, Stefan Hajnoczi wrote:
> > On Sat, Sep 18, 2010 at 10:27 PM, Edgar E. Iglesias
> > <edgar.iglesias@gmail.com> wrote:
> > > This doesn't look right. AFAIK, MAC's dont pad on receive.
> > 
> > I agree.  NICs that do padding will do it on transmit, not receive.
> > Anything coming in on the wire should already have the minimum length.
> > 
> > In QEMU that isn't true today and that's why rtl8139, pcnet, and
> > ne2000 already do this same padding.  This patch is the smallest
> > change to cover e1000.
> > 
> > > IMO this kind of padding should somehow be done by the bridge that forwards
> > > packets into the qemu vlan (e.g slirp or the generic tap bridge).
> > 
> > That should work and we can then drop the padding code from existing
> > NICs.  I'll take a look.
> > 
> > Stefan
> 
> Not all nic devices have to be emulate ethernet, so not all devices want
> the padding, e.g. virtio does not.

Right, ethernet behaviour should obviously not be applied unconditionally for
all net devices.


> It's also easy to imagine an
> ethernet device that strips the padding: would be silly to add it
> just to have it stripped.

I dont beleive that is possible. The FCS comes last, so an ethernet MAC
would have to do really silly things to differentiate between padding and
real payload.


> If we really want to do this generically, we could implement a function dealing
> with the padding, and call it from relevant devices.

Another way is to have network devices register their link types so that the
generic bridge can apply whatever link specific fixups that may be needed.

I would prefer to have the padding of bridged frames decoupled from the
device models, but I cant say I feel very strongly about this.

Cheers