Re: [PATCH] IB/ipoib: CSUM support in connected mode

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

On Thu, Jul 30, 2015 at 11:46:36PM +0300, Yuval Shaia wrote:
> On Thu, Jul 30, 2015 at 11:15:38AM -0600, Jason Gunthorpe wrote:
> > On Thu, Jul 30, 2015 at 11:51:12AM -0400, Doug Ledford wrote:
> > 
> > > In its current state, I have my doubts about this patch.  However, it
> > > seems to me that this should be relatively easy to fix in such a way
> > > that you get 90%+ of the performance benefit, and can turn it on by
> > > default, and we don't cause any problems.
> > 
> > The best way to implement this is to leverage all the checksum
> > offload work people did for virtualization.
> > 
> > Forward the checksum offload status through the RC connection and
> > recover it on the other side.
> The current approach is to utilize IPoIB's private-data to exchange this
> information.

You need private-data exchange to negotiate the feature.

The feature should be a per-packet csum status header.

When sending a skb that is already fully csumed the receiver sets
CHECKSUM_UNNECESSARY.

When sending a skb that has CHECKSUM_PARTIAL then the
receiver needs to call skb_partial_csum_set.

Look at how something like VIRTIO_NET_HDR_F_NEEDS_CSUM works and copy
that scheme.

DO NOT EVER set CHECKSUM_UNNECESSARY on packets that do not have valid
csums - that breaks the net stack.

Yes, you need to add a header to all packets to support this scheme,
that is what the private-data negotiation is for.

While you are at it, I'd make room for something like
VIRTIO_NET_HDR_GSO_* in the RC protocol too. Implementing GSO
forwarding is probably another big performance win.

Jason
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