Jakub Kicinski [off-list ref] writes:

On Wed, 22 Sep 2021 00:20:19 +0200 Toke Høiland-Jørgensen wrote:

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Neither of those are desirable outcomes, I think; and if we add a
separate "XDP multi-buff" switch, we might as well make it system-wide?  

If we have an internal flag 'this driver supports multi-buf xdp' cannot we
make xdp_redirect to linearize in case the packet is being redirected
to non multi-buf aware driver (potentially with corresponding non mb aware xdp
progs attached) from mb aware driver?  

Hmm, the assumption that XDP frames take up at most one page has been
fundamental from the start of XDP. So what does linearise mean in this
context? If we get a 9k packet, should we dynamically allocate a
multi-page chunk of contiguous memory and copy the frame into that, or
were you thinking something else?

My $.02 would be to not care about redirect at all.

It's not like the user experience with redirect is anywhere close 
to amazing right now. Besides (with the exception of SW devices which
will likely gain mb support quickly) mixed-HW setups are very rare.
If the source of the redirect supports mb so will likely the target.

It's not about device support it's about XDP program support: If I run
an MB-aware XDP program on a physical interface and redirect the (MB)
frame into a container, and there's an XDP program running inside that
container that isn't MB-aware, bugs will ensue. Doesn't matter if the
veth driver itself supports MB...

We could leave that as a "don't do that, then" kind of thing, but that
was what we were proposing (as the "do nothing" option) and got some
pushback on, hence why we're having this conversation :)

-Toke