[ofa-general] [PATCHv2] IB/ipoib: S/G and HW checksum support

[Jason Gunthorpe]

On Thu, Sep 06, 2007 at 08:12:23PM +0300, Michael S. Tsirkin wrote:

> > FWIW, general gateways do have a bit of a problem doing the csum
> > insertion because there are alot of cases and new protocols do crop up
> > from time to time. It would be best if part of the information sent in
> > this case was instructions on how to do the insertion like an general
> > ethernet chip would use.
> 
> Not sure I know what do you mean. Could you give an example please?

Ok, this is basically what the comments in skbuff.h about
NETIF_F_HW_CSUM vs NETIF_F_IP_CSUM are about, it applies just as well
to this case as to a NIC case.

Look at how skb_copy_and_csum_dev/skb_checksum_help works for
generic csumming.

Basically, the trick is the hw csum operates over a specified subset
of the packet. The csum value to update must be within that subset and
its location is also specified in the skb (csum_offset). The kernel
computes the IP pseudo header (or really any other additional bits
that get csumed) in advance and initializes the csum field in the
packet.

The HW then csums the range *which includes the csum field* and then
replaces the csum field with this new value. The results in the
hardware computing csum(pseudo_hdr) + csum(payload) without actually
having any idea what the pseudo_hdr is. This is generic non-protocol
specific offload (NETIF_F_HW_CSUM).

This is how CHECKSUM_PARTIAL works. CHECKSUM_COMPLETE is the analog on
the RX side. The HW computes a csum across every byte of the packet
and stores that out of band. Again through the properties of the csum
you can subtract bytes you don't want summed (more or less the csum of
the negative of the psuedo_hdr) from the csum and get a MAGIC constant
back if the packet csum is valid. This is how protocol agnostic
recieve csum offload is done.

Adding this to the IPoIB 'VNIC' wire protocol would relieve anyone
from actually having to figure out the pseudo_hdr and L4 protocol to
deduce the proper algorithm for computing the csum (NETIF_F_IP_CSUM)

Jason