On Wed, Jul 13, 2016 at 11:24:13AM +0200, Daniel Borkmann wrote: > Hi Peter, > > On 07/13/2016 09:52 AM, Peter Zijlstra wrote: > >On Wed, Jul 13, 2016 at 12:36:17AM +0200, Daniel Borkmann wrote: > >>This patch adds support for non-linear data on raw records. It means > >>that for such data, the newly introduced __output_custom() helper will > >>be used instead of __output_copy(). __output_custom() will invoke > >>whatever custom callback is passed in via struct perf_raw_record_frag > >>to extract the data into the ring buffer slot. > >> > >>To keep changes in perf_prepare_sample() and in perf_output_sample() > >>minimal, size/size_head split was added to perf_raw_record that call > >>sites fill out, so that two extra tests in fast-path can be avoided. > >> > >>The few users of raw records are adapted to initialize their size_head > >>and frag data; no change in behavior for them. Later patch will extend > >>BPF side with a first user and callback for this facility, future users > >>could be things like XDP BPF programs (that work on different context > >>though and would thus have a different callback), etc. > > > >Why? What problem are we solving? > > I've tried to summarize it in patch 3/3,
Which is pretty useless if you're staring at this patch. > This currently has 3 issues we'd like to resolve: > i) We need two copies instead of just a single one for the skb data. > The data can be non-linear, see also skb_copy_bits() as an example for > walking/extracting it, I'm not familiar enough with the network gunk to be able to read that. But upto skb_walk_frags() it looks entirely linear to me. > ii) for static verification reasons, the bpf_skb_load_bytes() helper > needs to see a constant size on the passed buffer to make sure BPF > verifier can do its sanity checks on it during verification time, so > just passing in skb->len (or any other non-constant value) wouldn't > work, but changing bpf_skb_load_bytes() is also not the real solution > since we still have two copies we'd like to avoid as well, and > iii) bpf_skb_load_bytes() is just for rather smaller buffers (e.g. > headers) since they need to sit on the limited eBPF stack anyway. The > set would improve the BPF helper to address all 3 at once. Humm, maybe. Lemme go try and reverse engineer that patch, because I'm not at all sure wth it's supposed to do, nor am I entirely sure this clarified things :/
