On Wed, Mar 01, 2023 at 03:53:47PM +0100, Toke Høiland-Jørgensen wrote: > Daniel P. Berrangé <berra...@redhat.com> writes: > > > On Tue, Feb 28, 2023 at 11:21:56PM +0100, Toke Høiland-Jørgensen wrote: > >> Daniel P. Berrangé <berra...@redhat.com> writes: > >> > >> > On Tue, Feb 28, 2023 at 08:01:51PM +0100, Toke Høiland-Jørgensen wrote: > >> >> Daniel P. Berrangé <berra...@redhat.com> writes: > >> >> > >> >> Just to interject a note on this here: the skeleton code is mostly a > >> >> convenience feature used to embed BPF programs into the calling binary. > >> >> It is perfectly possible to just have the BPF object file itself reside > >> >> directly in the file system and just use the regular libbpf APIs to load > >> >> it. Some things get a bit more cumbersome (mostly setting values of > >> >> global variables, if the BPF program uses those). > >> >> > >> >> So the JSON example above could just be a regular compiled-from-clang > >> >> BPF object file, and the management program can load that, inspect its > >> >> contents using the libbpf APIs and pass the file descriptors on to Qemu. > >> >> It's even possible to embed version information into this so that Qemu > >> >> can check if it understands the format and bail out if it doesn't - just > >> >> stick a version field in the configuration map as the first entry :) > >> > > >> > If all you have is the BPF object file is it possible to interrogate > >> > it to get a list of all the maps, and get FDs associated for them ? > >> > I had a look at the libbpf API and wasn't sure about that, it seemed > >> > like you had to know the required maps upfront ? If it is possible > >> > to auto-discover everything you need, soley from the BPF object file > >> > as input, then just dealing with that in isolation would feel simpler. > >> > >> It is. You load the object file, and bpf_object__for_each_map() lets you > >> discover which maps it contains, with the different bpf_map__*() APIs > >> telling you the properties of that map (and you can modify them too > >> before loading the object if needed). > >> > >> The only thing that's not in the object file is any initial data you > >> want to put into the map(s). But except for read-only maps that can be > >> added by userspace after loading the maps, so you could just let Qemu do > >> that... > >> > >> > It occurrs to me that exposing the BPF program as data rather than > >> > via binary will make more practical to integrate this into KubeVirt's > >> > architecture. In their deployment setup both QEMU and libvirt are > >> > running unprivileged inside a container. For any advanced nmetworking > >> > a completely separate component creates the TAP device and passes it > >> > into the container running QEMU. I don't think that the separate > >> > precisely matched helper binary would be something they can use, but > >> > it might be possible to expose a data file providing the BPF program > >> > blob and describing its maps. > >> > >> Well, "a data file providing the BPF program blob and describing its > >> maps" is basically what a BPF .o file is. It just happens to be encoded > >> in ELF format :) > >> > >> You can embed it into some other data structure and have libbpf load it > >> from a blob in memory as well as from the filesystem, though; that is > >> basically what the skeleton file does (notice the big character string > >> at the end, that's just the original .o file contents). > > > > Ok, in that case I'm really wondering why any of this helper program > > stuff was proposed. I recall the rationale was that it was impossible > > for an external program to load the BPF object on behalf of QEMU, > > because it would not know how todo that without QEMU specific > > knowledge. > > I'm not sure either. Was there some bits that initially needed to be set > before the program was loaded (read-only maps or something)? Also, > upstream does encourage the use of skeletons for embedding into > applications, so it's not an unreasonable thing to start with if you > don't have the kind of deployment constraints that Qemu does in this > case. > > > It looks like we can simply expose the BPF object blob to mgmt apps > > directly and get rid of this helper program entirely. > > I believe so, yes. You'd still need to be sure that the BPF object file > itself comes from a trusted place, but hopefully it should be enough to > load it from a known filesystem path? (Sorry if this is a stupid > question, I only have a fuzzy idea of how all the pieces fit together > here).
It could be from a well known location on the filesystem, but might be better to make it possible to query it from QMP, which is mostly safe *provided* you've not yet started guest CPUs running. It could be queried at startup and then cached for future use. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
