From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jakub Kicinski Subject: Re: [RFC bpf-next 2/6] net: xdp: RX meta data infrastructure Date: Fri, 6 Jul 2018 14:33:58 -0700 Message-ID: <20180706143358.0240ff66@cakuba.netronome.com> References: <20180627024615.17856-1-saeedm@mellanox.com> <20180627024615.17856-3-saeedm@mellanox.com> <20180703230137.hdoy2fujz3x4oeij@ast-mbp.dhcp.thefacebook.com> <13f973a9937834ae8c10bfcc7d90909e94c543f1.camel@mellanox.com> <65b964eb-9ee1-9fd8-d54a-9290377eb1e4@iogearbox.net> <20180705101800.3c5d6af0@cakuba.netronome.com> <20180706163041.xstyfednmgho23m3@ast-mbp.dhcp.thefacebook.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: Daniel Borkmann , Saeed Mahameed , "saeedm@dev.mellanox.co.il" , "alexander.h.duyck@intel.com" , "netdev@vger.kernel.org" , Tariq Toukan , "john.fastabend@gmail.com" , "brouer@redhat.com" , "borkmann@iogearbox.net" , "peter.waskiewicz.jr@intel.com" To: Alexei Starovoitov Return-path: Received: from mail-qt0-f193.google.com ([209.85.216.193]:37272 "EHLO mail-qt0-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753612AbeGFVeD (ORCPT ); Fri, 6 Jul 2018 17:34:03 -0400 Received: by mail-qt0-f193.google.com with SMTP id a18-v6so11103967qtj.4 for ; Fri, 06 Jul 2018 14:34:03 -0700 (PDT) In-Reply-To: <20180706163041.xstyfednmgho23m3@ast-mbp.dhcp.thefacebook.com> Sender: netdev-owner@vger.kernel.org List-ID: On Fri, 6 Jul 2018 09:30:42 -0700, Alexei Starovoitov wrote: > On Thu, Jul 05, 2018 at 10:18:23AM -0700, Jakub Kicinski wrote: > > > > I'm also not 100% on board with the argument that "future" FW can > > reshuffle things whatever way it wants to. Is the assumption that > > future ASICs/FW will be designed to always use the "blessed" BTF > > format? Or will it be reconfigurable at runtime? > > let's table configuration of metadata aside for a second. > > Describing metedata layout in BTF allows NICs to disclose everything > NIC has to users in a standard and generic way. > Whether firmware is reconfigurable on the fly or has to reflashed > and hw powercycled to have new md layout (and corresponding BTF description) > is a separate discussion. > Saeed's proposal introduces the concept of 'offset' inside 'struct xdp_md_info' > to reach 'hash' value in metadata. > Essentially it's a run-time way to access 'hash' instead of build-time. > So bpf program would need two loads to read csum or hash field instead of one. > With BTF the layout of metadata is known to the program at build-time. > > To reiterate the proposal: > - driver+firmware keep layout of the metadata in BTF format (either in the driver > or driver can read it from firmware) > - 'bpftool read-metadata-desc eth0 > md_desc.h' command will query the driver and > generate normal C header file based on BTF in the given NIC > - user does #include "md_desc.h" and bpf program can access md->csum or md->hash > with direct single load out of metadata area in front of the packet > - llvm compiles bpf program and records how program is doing this md->csum accesses > in BTF format as well (the compiler will be keeping such records > for __sk_buff and all other structs too, but that's separate discussion) > - during sys_bpf(prog_load) the kernel checks (via supplied BTF) that the way the program > accesses metadata (and other structs) matches BTF from the driver, > so no surprises if driver+firmware got updated, but program is not recompiled > - every NIC can have their own layout of metadata and its own meaning of the fields, > but would be good to standardize at least a few common fields like hash Can I expose HW descriptors this way, though, or is the proposal to copy this data into the packet buffer? > Once this is working we can do more cool things with BTF. > Like doing offset rewriting at program load time similar to what we plan > to do for tracing. Tracing programs will be doing 'task->pid' access > and the kernel will adjust offsetof(struct task_struct, pid) during program load > depending on BTF for the kernel. > The same trick we can do for networking metadata. > The program will contain load instruction md->hash that will get automatically > adjusted to proper offset depending on BTF of 'hash' field in the NIC. > For now I'm proposing _not_ to go that far with offset rewriting and start > with simple steps described above. Why? :( Could we please go with the rewrite/driver helpers instead of impacting fast paths of the drivers yet again? This rewrite should be easier than task->pid, because we have the synthetic user space struct xdp_md definition. The flexibility and power of BPF rewrites/JITing is at our disposal to deliver maximum performance..