On Thu, Jun 24, 2021 at 2:23 PM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > > Michal Swiatkowski <michal.swiatkowski@xxxxxxxxxxxxxxx> writes: > > > On Tue, Jun 22, 2021 at 01:53:33PM +0200, Toke Høiland-Jørgensen wrote: > >> Michal Swiatkowski <michal.swiatkowski@xxxxxxxxxxxxxxx> writes: > >> > >> > On Wed, Jun 02, 2021 at 09:18:37AM -0700, Jakub Kicinski wrote: > >> >> On Tue, 01 Jun 2021 17:22:51 -0700 John Fastabend wrote: > >> >> > > If we do this, the BPF program obviously needs to know which fields are > >> >> > > valid and which are not. AFAICT you're proposing that this should be > >> >> > > done out-of-band (i.e., by the system administrator manually ensuring > >> >> > > BPF program config fits system config)? I think there are a couple of > >> >> > > problems with this: > >> >> > > > >> >> > > - It requires the system admin to coordinate device config with all of > >> >> > > their installed XDP applications. This is error-prone, especially as > >> >> > > the number of applications grows (say if different containers have > >> >> > > different XDP programs installed on their virtual devices). > >> >> > > >> >> > A complete "system" will need to be choerent. If I forward into a veth > >> >> > device the orchestration component needs to ensure program sending > >> >> > bits there is using the same format the program installed there expects. > >> >> > > >> >> > If I tailcall/fentry into another program that program the callee and > >> >> > caller need to agree on the metadata protocol. > >> >> > > >> >> > I don't see any way around this. Someone has to manage the network. > >> >> > >> >> FWIW I'd like to +1 Toke's concerns. > >> >> > >> >> In large deployments there won't be a single arbiter. Saying there > >> >> is seems to contradict BPF maintainers' previous stand which lead > >> >> to addition of bpf_links for XDP. > >> >> > >> >> In practical terms person rolling out an NTP config change may not > >> >> be aware that in some part of the network some BPF program expects > >> >> descriptor not to contain time stamps. Besides features may depend > >> >> or conflict so the effects of feature changes may not be obvious > >> >> across multiple drivers in a heterogeneous environment. > >> >> > >> >> IMO guarding from obvious mis-configuration provides obvious value. > >> > > >> > Hi, > >> > > >> > Thanks for a lot of usefull information about CO-RE. I have read > >> > recommended articles, but still don't understand everything, so sorry if > >> > my questions are silly. > >> > > >> > As introduction, I wrote small XDP example using CO-RE (autogenerated > >> > vmlinux.h and getting rid of skeleton etc.) based on runqslower > >> > implementation. Offset reallocation of hints works great, I built CO-RE > >> > application, added new field to hints struct, changed struct layout and > >> > without rebuilding application everything still works fine. Is it worth > >> > to add XDP sample using CO-RE in kernel or this isn't good place for > >> > this kind of sample? > >> > > >> > First question not stricte related to hints. How to get rid of #define > >> > and macro when I am using generated vmlinux.h? For example I wanted to > >> > use htons macro and ethtype definition. They are located in headers that > >> > also contains few struct definition. Because of that I have redefinition > >> > error when I am trying to include them (redefinition in vmlinux.h and > >> > this included file). What can I do with this besides coping definitions > >> > to bpf code? > >> > >> One way is to only include the structs you actually need from vmlinux.h. > >> You can even prune struct members, since CO-RE works just fine with > >> partial struct definitions as long as the member names match. > >> > >> Jesper has an example on how to handle this here: > >> https://github.com/netoptimizer/bpf-examples/blob/ktrace01-CO-RE.public/headers/vmlinux_local.h > >> > > > > I see, thanks, I will take a look at other examples. > > > >> > I defined hints struct in driver code, is it right place for that? All > >> > vendors will define their own hints struct or the idea is to have one > >> > big hints struct with flags informing about availability of each fields? > >> > > >> > For me defining it in driver code was easier because I can have used > >> > module btf to generate vmlinux.h with hints struct inside. However this > >> > break portability if other vendors will have different struct name etc, > >> > am I right? > >> > >> I would expect the easiest is for drivers to just define their own > >> structs and maybe have some infrastructure in the core to let userspace > >> discover the right BTF IDs to use for a particular netdev. However, as > >> you say it's not going to work if every driver just invents their own > >> field names, so we'll need to coordinate somehow. We could do this by > >> convention, though, it'll need manual intervention to make sure the > >> semantics of identically-named fields match anyway. > >> > >> Cf the earlier discussion with how many BTF IDs each driver might > >> define, I think we *also* need a way to have flags that specify which > >> fields of a given BTF ID are currently used; and having some common > >> infrastructure for that would be good... > >> > > > > Sounds good. > > > > Sorry, but I feel that I don't fully understand the idea. Correct me if > > I am wrong: > > > > In building CO-RE application step we can defined big struct with > > all possible fields or even empty struct (?) and use > > bpf_core_field_exists. > > > > bpf_core_field_exists will be resolve before loading program by libbpf > > code. In normal case libbpf will look for btf with hints name in vmlinux > > of running kernel and do offset rewrite and exsistence check. But as the > > same hints struct will be define in multiple modules we want to add more > > logic to libbpf to discover correct BTF ID based on netdev on which program > > will be loaded? > > I would expect that the program would decide ahead-of-time which BTF IDs > it supports, by something like including the relevant structs from > vmlinux.h. And then we need the BTF ID encoded into the packet metadata > as well, so that it is possible to check at run-time which driver the > packet came from (since a packet can be redirected, so you may end up > having to deal with multiple formats in the same XDP program). > > Which would allow you to write code like: > > if (ctx->has_driver_meta) { > /* this should be at a well-known position, like first (or last) in meta area */ > __u32 *meta_btf_id = ctx->data_meta; > > if (*meta_btf_id == BTF_ID_MLX5) { > struct meta_mlx5 *meta = ctx->data_meta; > /* do something with meta */ > } else if (meta_btf_id == BTF_ID_I40E) { > struct meta_i40e *meta = ctx->data_meta; > /* do something with meta */ > } /* etc */ > } > > and libbpf could do relocations based on the different meta structs, > even removing the code for the ones that don't exist on the running > kernel. Just wondering how this will carry over to user-space and AF_XDP since it sees the same metadata area as XDP? AFAIK, dynamic linkers today cannot relocate structs or remove members, but I am not up-to-date with the latest here so might be completely wrong. And it would be good not to have to recompile a user-space binary just because a new NIC came out with a new BTF ID and layout, but with the same metadata member name and format as previous NICs/BTF IDs. But I do not know how to solve these things in user-space at the moment (except to have fixed locations for a common set of metadata, but that is what we are trying to avoid), so any hints and suggestions are highly appreciated. > -Toke >
