Re: ib_ipoib: CSUM support in connected mode

[Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>]

On Mon, Sep 15, 2014 at 05:47:19PM +0300, Or Gerlitz wrote:

> > [...] The proposal is to tell to network stack that IPoIB-CM supports IP
> > Checksum offload. This enables Linux IPoIB-CM driver to use Scatter/Gather feature. Network
> > sends the IP packet without adding the IP Checksum to the header.
> 
> AFAIK, on the TX side, Linux will always compute the IP checksum,
> but with this suggestion, not the TCP checksum which is assumed to
> be computed by the card... so we will have a TCP packet on the wire
> without checksum. And if this packet goes through gateway it will be
> dropped at some point, agree?

I remember this was discussed a few years ago on this list.

To do this, you need to transfer the offload state across the wire, so
on receive you inject the packet with the proper tag that the csum is
not computed but ready for offload. A node receiving a packet like
this would have to compute the csum before sending it onwards, so no,
if done properly it will not break gateways.

All the core infrastructure is there, all the virtualization drivers
work like this - the guest side does not compute the csum, and the
hyperviser side receives the packet with that flag, and the csum
ultimately is offloaded to the physical NIC. Look at the xen net
driver for an example.

The main thing is to negotiate this and other features at RC
connection time. Be sure to leave room for other optimizations, for
instance IPOIB could forward a GSO packet unbroken to the remote side.

Jason
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