Zvi Effron via xdp-hints <xdp-hints@xxxxxxxxxxxxxxx> writes: > On Thu, Jun 24, 2021 at 5:23 AM 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. >> >> -Toke >> > > How does putting the BTF ID and the driver metadata into the XDP metadata > section interact with programs that are already using the metadata section > for other purposes. For example, programs that use the XDP metadata to pass > information through BPF tail calls? > > Would this break existing programs that aren't aware of the new driver > metadata? Do we need to make driver metadata opt-in at XDP program > load? Well, XDP applications would be free to just ignore the driver-provided metadata and overwrite it with its own data? And I guess any application that doesn't know about it will just implicitly do that? :) -Toke
