RE: [Xen-devel] [PATCH 4/6] xen/netback: Always pull throughPKT_PROT_LEN bytes into the linear part of an skb.

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> 
> On Wed, 2010-02-24 at 08:28 +0000, Jan Beulich wrote:
> > Could you point out what problem this addresses?
> 
> It ensures that at least the TCP/IP headers will be pulled into the
> linear part of the SKB. At least skb_checksum_setup relies on this and
I
> think it is a more generic assumption in at least some parts of the
> network stack as well. The next patch increases PKT_PROT_LEN to
include
> the TCP options as well since we have observed cases where Windows
> guests with PV drivers can generate a frame with a split at the point.
> 

My PV drivers couldn't always be relied on to do this, because Windows
couldn't be relied on to do it either. I just coalesced the first buffer
to some minimum value depending on the header type. If you are doing TCP
checksum offload then you have to tell Windows that you support IP
checksum offload too - Linux doesn't support that so you have to lie to
Windows about it and calculate the IP checksum in the PV drivers, and if
you are doing that you have to have your own copy of the header anyway
so it turns out not to be a big deal.

James


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