On Tue, Sep 14, 2021 at 5:23 AM Larysa Zaremba <larysa.zaremba@xxxxxxxxx> wrote: > > From: Jesper Dangaard Brouer <jbrouer@xxxxxxxxxx> > Date: Mon, 13 Sep 2021 13:35:04 +0200 > > > Trying to get started with XDP-hints again. > > > The fundamental idea is that XDP-hints metadata struct's are defined by > > the kernel, preferably by the kernel module, and described via BTF. As > > BTF layout is defined by kernel, this means userspace (AF_XDP) and > > BPF-progs must adapt to layout (used by running kernel). This imply that > > kernel is free to change layout. > > > The BTF ID is exposed (to BPF-prog and AF_XDP) on per packet basis, to > > give kernel more freedom to change layout runtime. This push > > responsibility to userspace/BPF-prog of handling different layouts, > > which seems natural. For the kernel this solves many issues around > > concurrency and NIC config changes that affects BTF info available (e.g. > > when BTF layout is allowed to change). > > Aside from generic XDP hints defined directly in kernel code, > it should be possible to define custom hints layout inside network driver. > Kernel module BTF support[1][2] provides the means to do so. > > As long as our XDP program is attached to interfaces serviced by a single driver > and BPF program uses hints layouts only from this particular driver + generic, > there should be no problem, we can easily obtain a BTF type_id in BPF code > and compare it to the one sent by driver, > because BTF type_ids are unique in such case (was discussed earlier). > > However, if we want a single program to use layouts from multiple drivers, > we can’t do it so easily, because in such case same type_id can point > to absolutely different types. Obviously, probability of both those types being > hints structures is extremely low, > but using type_id only would be at least inconsistent. > This generates the following issues: > > 1. We have to send both BTF ID and type_id in generic hints struct from driver. > This takes up precious space inside generic structure. > The first solution we’ve came up with is not using full 64 bits, but instead > splitting 32 bits we already planned for type_id into for example, > 12 for BTF_ID and 20 for type_id. Feel free to suggest a better solution. > 2. BTF ID needs to be compared in BPF code too, > but it’s not that easily obtained as type_id > (through bpf_core_type_id_kernel which expands to __builtin_btf_type_id). > This patch[3] suggests that at least at some point there was an intension > to return both BTF_ID and type_id in a single 64 bit integer. > However, I’ve run a test and bpf_core_type_id_kernel does not seem > to return anything on the most significant 32 bits. > libbpf source also seems to patch only type_ids into program. > Therefore I have a question to BPF CO-RE developers, > do I understand the situation correctly? > Is there any other existing way to obtain a BTF ID (not type_id) > from type name inside BPF code? I've even posted a patch before that was resolving local BTF types to the module BTF FD/ID (that was one of the points of contention) and BTF type ID. See [0] for context. I think we never reached any conclusion and this never landed. [0] https://patchwork.kernel.org/project/netdevbpf/patch/20201205025140.443115-1-andrii@xxxxxxxxxx/ > > So I would like to ask for some suggestions in this situation. > > > > End-goal is to make it easier for kernel drivers can invent new layouts > > to suite new hardware features. Thus, we prefer a solution where > > XDP-hints metadata struct's are defined in the kernel module code. > > > > (Idea below ... please let us know what you think, wrong direction?) > > > Exploring kernel module code defining the XDP-hints metadata struct. > > > Kernel module BTFs are now[1][2] exposed through sysfs as > > /sys/kernel/btf/<module-name>. Thus, userspace can use libbpf > > btf__load_module_btf() and others BTF APIs. Started playing here[3]. > > > Credit to Toke, who had an idea that drivers could "say" what struct's > > are available, by defining a union with a known name e.g. > > metadata_hints_avail' and have supported metadata struct's included in > > that union. Then we don't need new APIs for exporting these BTF-metadata > > struct's. To find struct names, we BTF walk this union. > > > > -Jesper > > --Larysa > > > [1] > > https://lore.kernel.org/bpf/20201110011932.3201430-5-andrii@xxxxxxxxxx/ > > [2] 36e68442d1af ("bpf: Load and verify kernel module BTFs") (Author: > > Andrii Nakryiko) > > [3] > > https://github.com/xdp-project/bpf-examples/blob/master/BTF-playground/ > > [1] > https://lwn.net/Articles/836249/ > [2] > https://www.spinics.net/lists/netdev/msg702049.html > [3] > https://reviews.llvm.org/D91489 > > ________________________________ > Intel Technology Poland sp. z o.o. > ul. Słowackiego 173 | 80-298 Gdańsk | Sąd Rejonowy Gdańsk Północ | VII Wydział Gospodarczy Krajowego Rejestru Sądowego - KRS 101882 | NIP 957-07-52-316 | Kapitał zakładowy 200.000 PLN. > > Ta wiadomość wraz z załącznikami jest przeznaczona dla określonego adresata i może zawierać informacje poufne. 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