On Wed, 2007-05-09 at 20:12 +1000, Herbert Xu wrote:

akpm@linux-foundation.org wrote:

quoted

+       if (desc->features & LGUEST_NET_F_NOCSUM)
+               dev->features |= NETIF_F_NO_CSUM;

Any reason why you're using NO_CSUM here instead of HW_CSUM?
Practically there is no difference but NO_CSUM could be treated
differently in future and I'm not sure whether such changes would
be desirable in this driver (i.e., not even generating a partial
checksum).

Hi Herbert,

	NO_CSUM because it really doesn't need a checksum.  The
LGUEST_NET_F_NOCSUM is only set for local inter-guest networking.  If
some guest were to route the packets outside the machine, this would be
an issue, though ("don't do that").

Also, there doesn't seem to be any passing of metadata to the
backend which means that neither NO_CSUM/HW_CSUM can work if
somebody needs to look at the checksum field.  You could use
IP_CSUM if you do the same hack on the backend that Xen does.
Otherwise you'll have to make do with no checksum offload at all.

Yeah, definitely good future work.  This is far simpler: external-facing
networks don't get marked with the LGUEST_NET_F_NOCSUM but set.

KVM is going to have a paravirt network driver too: it'd be nice to
share code here.

I think you'd also need a change_mtu function if the SG feature
is going to be of some use since the default Ethernet one limits
the MTU to 1500.

Indeed, that would be a new feature, and is certainly a consideration
for more efficient inter-guest networking.  However, I consider that
somewhat cheating: it's nice to know that 1500 doesn't suck too hard.

Remember, "Features kill puppies!" 8)

Thanks,
Rusty.