Re: [PATCH v9 bpf-next 08/14] bpf: add multi-buff support to the bpf_xdp_adjust_tail() API

Lorenzo Bianconi [off-list ref] writes:

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Eelco Chaudron wrote:

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On 23 Jun 2021, at 1:37, John Fastabend wrote:

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Lorenzo Bianconi wrote:

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From: Eelco Chaudron <echaudro@redhat.com>

This change adds support for tail growing and shrinking for XDP multi-buff.

It would be nice if the commit message gave us some details on how the
growing/shrinking works in the multi-buff support.

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Guess this is the tricky part, applications need to be multi-buffer aware. If current applications rely on bpf_xdp_adjust_tail(+) to determine maximum frame length this approach might not work. In this case, we might need an additional helper to do tail expansion with multi buffer support.

But then the question arrives how would mb unaware application behave in general when an mb packet is supplied?? It would definitely not determine the correct packet length.

Right that was my conclusion as well. Existing programs might
have subtle side effects if they start running on multibuffer
drivers as is. I don't have any good ideas though on how
to handle this.

what about checking the program capabilities at load time (e.g. with a
special program type) and disable mb feature if the bpf program is not
mb-aware? (e.g. forbid to set the MTU greater than 1500B in xdp mode).

So what happens when that legacy program runs on a veth and gets an
mb-enabled frame redirected into it? :)

-Toke