On Thu, 24 Jun 2021 00:10:12 +0000 "Desouza, Ederson" <ederson.desouza@xxxxxxxxx> wrote: > Following current discussions around XDP hints, it's clear that > currently the focus is on BPF applications. But my interest is in the > AF_XDP side of things - user space applications. I agree, that most of the discussion is focused on BPF-programs being loaded into the kernel via libbpf. I actually also care about getting this working for AF_XDP. We've discussed this with Magnus (meeting yesterday) and I think we agree that this is also something we want for AF_XDP. IIRC the plan is to use one bit to indicate if a packet is carrying info in metadata area, as (1) AF_XDP descriptor don't have room for storing the BTF-ID, and (2) if bit is not set, then we can avoid touching that cache-line. If the bit is set, then the BTF-ID is stored in metadata area (preferably as the last member, as ctx->data_meta is a minus offset from ctx->data, making it accessible via a fixed offset from data). For the BPF-programs it would make sense to store the BTF-ID in xdp_buff/xdp_frame and make it accessible via xdp_md (ctx seen from BPF-prog). To help AF_XDP the *proposal* is to (also) store it in metadata area itself. > In there, there's not much help from BPF CO-RE - who's going to rewrite > user space structs, after all? Well, AFAIK most of the offset relocation happens in user-space by libbpf. Which Alexei also indicate in the other thread[1]. To better understand BTF/CO-RE I've coded up an example here[2]. [1] https://lore.kernel.org/bpf/CAADnVQKv5SLBfnBWnEBFqf0-DQv+NZuixGiCVx1hewfQFhHSKg@xxxxxxxxxxxxxx/ [2] https://github.com/xdp-project/bpf-examples/blob/master/ktrace-CO-RE/ktrace01_kern.c I'm trying to understand how libbpf does this. So, I added a --debug option that makes libbpf print verbose messages. See commit[3] that also contains output example. [3] https://github.com/xdp-project/bpf-examples/commit/0542d8a7a327b642d105 Some of the --debug output: libbpf: loading kernel BTF '/sys/kernel/btf/vmlinux': 0 [...] libbpf: CO-RE relocating [0] struct sk_buff___local: found target candidate [2965] struct sk_buff in [vmlinux] libbpf: prog 'udp_send_skb': relo #1: matching candidate #0 [2965] struct sk_buff.hash (0:55 @ offset 148) libbpf: prog 'udp_send_skb': relo #1: patched insn #1 (ALU/ALU64) imm 4 -> 148 libbpf: prog 'udp_send_skb': relo #2: kind <byte_off> (0), spec is [7] struct sk_buff___local.len (0:0 @ offset 0) libbpf: prog 'udp_send_skb': relo #2: matching candidate #0 [2965] struct sk_buff.len (0:6 @ offset 112) libbpf: prog 'udp_send_skb': relo #2: patched insn #8 (ALU/ALU64) imm 0 -> 112 libbpf: prog 'udp_send_skb': relo #3: kind <target_type_id> (7), spec is [7] struct sk_buff___local libbpf: prog 'udp_send_skb': relo #3: matching candidate #0 [2965] struct sk_buff libbpf: prog 'udp_send_skb': relo #3: patched insn #24 (ALU/ALU64) imm 7 -> 2965 As indicated in [1] a BTF matching is being done in userspace. First libbpf loads kernels BTF from '/sys/kernel/btf/vmlinux'. Then it have the BTF from BPF-prog 'sk_buff___local' which finds target 'struct sk_buff' as btf_id 2965. Afterwards it patches the relocations in the byte code. > So, I decided to give a try at a possible implementation, using igc > driver as I'm more used to it, and come here ask some questions about > it. > > For the curious, here's my branch with current work: > > https://github.com/edersondisouza/linux/tree/xdp-hints > > It's on top of Alexandr Lobakin and Michal Swiatkowski work - but I > decided to incorporate some of the CO-RE related feedback, so I could > have something that also works with BPF applications. Please not that > I'm not trying to jump ahead of them in incorporating the feedback - > probably they have something more robust here - but if you see some > value in my patches, feel free to reuse/incorporate them (if they are > just an example of what not to do, it's still an example =D ). > I also added some XDP ZC patches for igc that are still moving to > mainline. > > In there, I basically defined a sample of "generic hints", that is > basically an struct with common hints, such as RX and TX timestamp, > hash, etc. I also included two more members to that struct: field_map > and extension_id. The first, shows which members are actually valid in > the data, the second is an arbitrary id that drivers can use to say > "there's extra data" beyond the generic members, and how to interpret > what's there is driver specific. A BTF is also created to represent > this struct, and registering is done the same way Saeed's patch did. > > User space developers that need to get the struct can use something > like to get it from the driver: > > # tools/bpf/bpftool/bpftool net xdp show > xdp: > enp6s0(5) md_btf_id(60) md_btf_enabled(1) > > And use the btf_id to get the struct: > > # bpftool btf dump file /sys/kernel/btf/igc format c > > Currently though, that's bad - as in this case the struct has no > types, only the field names. Why? I don't follow, what is not working? > With the driver specific struct (or by using the generic one, if no > specific fields are needed), the application can then access the XDP > frame metadata. I've also added some helpers to aid getting the > metadata. > > I added some examples on how to use those (they may be too simplistic), > so it's possible to get a feel on how this API might work. > > My goals for this email are to check if this approach is valid and what > pitfalls can you see. I didn't send a patch series yet to not jump > ahead Alexandr and Michal work (I can rebase on top of their work > later) and because the igc RX and TX timestamp implementation I'm using > to provide more real looking data is not yet complete. > > Another goal is to ensure that AF_XDP side is not forgotten in the XDP > hints discussion =D Thanks for pointing that out :-) > Naturally, if someone finds any issue trying those patches, please let > me know! -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
