On Fri, Jul 06, 2018 at 02:33:58PM -0700, Jakub Kicinski wrote: > 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?
That crossed my mind too. We can use BTF to describe HW descriptors too, but I don't think it would buy us anything. AF_XDP approach is better. > > 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. I don't understand 'impact fast path yet again' concern. If NIC has certain metadata today, just derscribe what it has in BTF and supply the actual per-packet md to xdp program as-is. No changes for fast path at all. Future rewritting will be done by the bpf/xdp core with zero changes to the driver. All driver has to do is to provide BTF.
