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

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

On Wed, Nov 18, 2015 at 10:27:41PM +0200, Yuval Shaia wrote:
> > 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.

> Correct me if i'm wrong here but isn't this protocol assume both parties
> are aware of this special header?

Yes.

No matter what you do both sides must be aware of the change in
protocol, doing it correctly requires adding a small wire header to
flow through the checksum state. This would be enabled once
negotiation confirms both sides will support this.

> My case is a bit different, driver must support backward
> computability in a way that peer maybe a driver that do not support
> this feature and expect packet to be full checksummed.

This is why negotiation is mandatory.

> > DO NOT EVER set CHECKSUM_UNNECESSARY on packets that do not have valid
> > csums - that breaks the net stack.
> The entire idea here is to fake csum offload so how would i tell the stack
> not to run csum on incoming packet?

I already explained this, call skb_partial_csum_set and the stack will
avoid csum work.

> > 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.
> If i understood you correctly and you mean exchange of
> "driver-capabilities", then yes, it is there with the extends of
> ipoib_cm_data structure.

You need to dig into how the things I referenced above work, fully
understand them and then adapt them to IPoIB.

Jason
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