Lorenzo Bianconi wrote:

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In general I see no reason to populate these fields before the XDP
program runs. Someone needs to convince me why having frags info before
program runs is useful. In general headers should be preserved and first
frag already included in the data pointers. If users start parsing further
they might need it, but this series doesn't provide a way to do that
so IMO without those helpers its a bit difficult to debate.

We need to populate the skb_shared_info before running the xdp program in order to
allow the ebpf sanbox to access this data. If we restrict the access to the first
buffer only I guess we can avoid to do that but I think there is a value allowing
the xdp program to access this data.

I agree. We could also only populate the fields if the program accesses
the fields.

A possible optimization can be access the shared_info only once before running
the ebpf program constructing the shared_info using a struct allocated on the
stack.

Seems interesting, might be a good idea.

Moreover we can define a "xdp_shared_info" struct to alias the skb_shared_info
one in order to have most on frags elements in the first "shared_info" cache line.

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Specifically for XDP_TX case we can just flip the descriptors from RX
ring to TX ring and keep moving along. This is going to be ideal on
40/100Gbps nics.

I'm not arguing that its likely possible to put some prefetch logic
in there and keep the pipe full, but I would need to see that on
a 100gbps nic to be convinced the details here are going to work. Or
at minimum a 40gbps nic.

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Not against it, but these things are a bit tricky. Couple things I still
want to see/understand

 - Lets see a 40gbps use a prefetch and verify it works in practice
 - Explain why we can't just do this after XDP program runs

how can we allow the ebpf program to access paged data if we do not do that?

I don't see an easy way, but also this series doesn't have the data
access support.

Its hard to tell until we get at least a 40gbps nic if my concern about
performance is real or not. Prefetching smartly could resolve some of the
issue I guess.

If the Intel folks are working on it I think waiting would be great. Otherwise
at minimum drop the helpers and be prepared to revert things if needed.

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 - How will we read data in the frag list if we need to parse headers
   inside the frags[].

The above would be best to answer now rather than later IMO.

Thanks,
John

Regards,
Lorenzo