On Mon, 05 Oct 2020 21:29:36 -0700
John Fastabend [off-list ref] wrote:

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Lorenzo Bianconi wrote:

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[...]
  

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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.

Notice, a driver will not initialize/use the shared_info area unless
there are more segments.  And (we have already established) the xdp->mb
bit is guarding BPF-prog from accessing shared_info area. 

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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.

It *might* be a good idea ("alloc" shared_info on stack), but we should
benchmark this.  The prefetch trick might be fast enough.  But also
keep in mind the performance target, as with large size frames the
packet-per-sec we need to handle dramatically drop.

right. I guess we need to define a workload we want to run for the
xdp multi-buff use-case (e.g. if MTU is 9K we will have ~3 frames
for each packets and # of pps will be much slower)

[...]

I do think it makes sense to drop the helpers for now, and focus on how
this new multi-buffer frame type is handled in the existing code, and do
some benchmarking on higher speed NIC, before the BPF-helper start to
lockdown/restrict what we can change/revert as they define UAPI.

ack, I will drop them in v5.

Regards,
Lorenzo

E.g. existing code that need to handle this is existing helper
bpf_xdp_adjust_tail, which is something I have broad up before and even
described in[1].  Lets make sure existing code works with proposed
design, before introducing new helpers (and this makes it easier to
revert).

[1] https://github.com/xdp-project/xdp-project/blob/master/areas/core/xdp-multi-buffer01-design.org#xdp-tail-adjust
-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer