Re: [patch 7/9] lguest: the net driver

Re: [patch 7/9] lguest: the net driver

[Herbert Xu]

akpm@linux-foundation.org wrote:
>
> +       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).

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.

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.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [patch 7/9] lguest: the net driver

[Rusty Russell]

On Wed, 2007-05-09 at 20:12 +1000, Herbert Xu wrote:
> akpm@linux-foundation.org wrote:
> >
> > +       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.

Re: [patch 7/9] lguest: the net driver

[Herbert Xu]

Hi Rusty:

On Wed, May 09, 2007 at 09:55:25PM +1000, Rusty Russell wrote:
> 
> 	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").

While I can see that this is good in keeping things simple, I think
it's something that you want to be able to support since the user
may wish to setup a guest as a firewall appliance which would involve
passing packets from another guest to the outside world.
 
> 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)

Heh :)

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [patch 7/9] lguest: the net driver

[Rusty Russell]

On Wed, 2007-05-09 at 22:00 +1000, Herbert Xu wrote:
> Hi Rusty:
> 
> On Wed, May 09, 2007 at 09:55:25PM +1000, Rusty Russell wrote:
> > 
> > 	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").
> 
> While I can see that this is good in keeping things simple, I think
> it's something that you want to be able to support since the user
> may wish to setup a guest as a firewall appliance which would involve
> passing packets from another guest to the outside world.

Indeed, you understand the tradeoff.  The example launcher could have an
option not to set the LGUEST_NET_F_NOCSUM in this case.

That said, one significant purpose of lguest is to serve as an example
of how to do things.  So if you feel really strongly that there's a
Right Way, we could look at the patch...

Thanks,
Rusty.