Daniel Borkmann <daniel@xxxxxxxxxxxxx> writes: > 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/ Right, well in that case maybe we should just expose the actual headroom as a separate netlink attribute? Although I suppose that would require another round of driver changes since Jesper's patch you linked above only puts this into xdp_buff at XDP program runtime. Jesper, WDYT? -Toke
