On 12/4/20 6:20 PM, Toke Høiland-Jørgensen wrote:
Daniel Borkmann <daniel@xxxxxxxxxxxxx> writes:
[...]
We tried to standardize on a minimum guaranteed amount, but unfortunately not
everyone seems to implement it, but I think it would be very useful to query
this from application side, for example, consider that an app inserts a BPF
prog at XDP doing custom encap shortly before XDP_TX so it would be useful to
know which of the different encaps it implements are realistically possible on
the underlying XDP supported dev.
How many distinct values are there in reality? Enough to express this in
a few flags (XDP_HEADROOM_128, XDP_HEADROOM_192, etc?), or does it need
an additional field to get the exact value? If we implement the latter
we also run the risk of people actually implementing all sorts of weird
values, whereas if we constrain it to a few distinct values it's easier
to push back against adding new values (as it'll be obvious from the
addition of new flags).
It's not everywhere straight forward to determine unfortunately, see also [0,1]
as some data points where Jesper looked into in the past, so in some cases it
might differ depending on the build/runtime config..
[0] https://lore.kernel.org/bpf/158945314698.97035.5286827951225578467.stgit@firesoul/
[1] https://lore.kernel.org/bpf/158945346494.97035.12809400414566061815.stgit@firesoul/
