Re: [PATCH net-next 2/2] bnx2x: add RSS capability for GRE traffic

[Eric Dumazet <eric.dumazet@gmail.com>]

Please Maciej do not top post on lkml or netdev mailing lists.

On Tue, 2013-03-19 at 02:18 -0700, Maciej Żenczykowski wrote:
> Can the HW calculate and return a 1s complement sum of the entire
> packet (or a large portion there-of)?
> Fixing that up to be only of the outer IPv4, inner IPv4 and inner TCP
> relevant portions should still be simpler (well faster) than
> calculating the TCP checksum.
> I'm pretty sure that some relationship between 1s complement sum of
> all bytes, outer IPv4 checksum, inner IPv4 checksum and TCP checksum
> could be pulled out of a hat with some deeper thought.  (similarly for
> IPv4/GRE/IPv6/TCP and other combinations)
> 
> What portions of the packet can the HW/FW [partially] checksum - and
> return the value to the driver for further processing?
> Can it return 1s complement sum of data portion of outer IPv4 (ie. in
> IPv4/GRE/IPv4/TCP return a 1s complement sum of GRE/IPv4/TCP bytes)
> 

I assume Dmitry was speaking of this possibility, and our stack should
handle this just fine.

NIC providing these kind of checksums set :

skb->ip_summed = CHECKSUM_COMPLETE;
skb->csum = csum;

before feeding the packet to the stack.

When we pull some header, we have to call skb_postpull_rcsum()
to adjust the skb->csum so that final check can be done.

About 20 drivers currently provide these kind of checksumming.