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AJvYcCUn/HJJbFACI6H1jlCU1IcJP7A5/mohzAwTdqArgf3dfG/xGEvbRtk54YlRnChAfC2P5f0=@vger.kernel.org X-Gm-Message-State: AOJu0YzxjUJFt6nZkFXVEuoqcnqrD3Cv3tY2sXJ6pconVSBroMLrUr7p A3Mq5yCE+oV3kF+dh5I9oiIjetUPBEd6DmW5C5GaWbr8K2c+MRlLtMLwjvfrTmY= X-Google-Smtp-Source: AGHT+IFbKmOHQzYH6RU5RJ695hEsh3LWNKXU2AyvEHj2fyjWXajYvlxmm7C65l5t2tAIDZnqYBHP2Q== X-Received: by 2002:a05:6000:1141:b0:374:b9a7:5ed6 with SMTP id ffacd0b85a97d-37cd5ab7652mr9443619f8f.22.1727791581955; Tue, 01 Oct 2024 07:06:21 -0700 (PDT) Received: from localhost ([2a09:bac1:27c0:58::241:6e]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-37cd5742499sm11766826f8f.93.2024.10.01.07.06.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 01 Oct 2024 07:06:21 -0700 (PDT) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Tue, 01 Oct 2024 16:06:20 +0200 Message-Id: To: =?utf-8?q?Toke_H=C3=B8iland-J=C3=B8rgensen?= Cc: "Lorenzo Bianconi" , "Jesper Dangaard Brouer" , "Jakub Sitnicki" , "Alexander Lobakin" , "Lorenzo Bianconi" , , , , , , , , , , , , , , , , , , , , "kernel-team" , "Yan Zhai" Subject: Re: [RFC bpf-next 0/4] Add XDP rx hw hints support performing XDP_REDIRECT From: "Arthur Fabre" X-Mailer: aerc 0.8.2 References: <1f53cd74-6c1e-4a1c-838b-4acc8c5e22c1@intel.com> <09657be6-b5e2-4b5a-96b6-d34174aadd0a@kernel.org> <87ldzkndqk.fsf@toke.dk> <87wmiysi37.fsf@toke.dk> <87ldzds8bp.fsf@toke.dk> <874j5xs9b1.fsf@toke.dk> In-Reply-To: <874j5xs9b1.fsf@toke.dk> On Mon Sep 30, 2024 at 12:52 PM CEST, Toke H=C3=B8iland-J=C3=B8rgensen wrot= e: > > Thinking about it more, my only relectance for a registration API is ho= w > > to communicate the ID back to other consumers (our discussion below). > > > >> > >> > Dynamically registering fields means you have to share the returned = ID > >> > with whoever is interested, which sounds tricky. > >> > If an XDP program sets a field like packet_id, every tracing > >> > program that looks at it, and userspace service, would need to know = what > >> > the ID of that field is. > >> > Is there a way to easily share that ID with all of them? > >> > >> Right, so I'll admit this was one of the handwavy bits of my original > >> proposal, but I don't think it's unsolvable. You could do something li= ke > >> (once, on application initialisation): > >> > >> __u64 my_key =3D bpf_register_metadata_field(my_size); /* maybe add a = name for introspection? */ > >> bpf_map_update(&shared_application_config, &my_key_index, &my_key); > >> > >> and then just get the key out of that map from all programs that want = to > >> use it? > > > > Passing it out of band works (whether it's via a pinned map like you > > described, or through other means like a unix socket or some other > > API), it's just more complicated. > > > > Every consumer also needs to know about that API. That won't work with > > standard tools. For example if we set a PACKET_ID KV, maybe we could > > give it to pwru so it could track packets using it? > > Without registering keys, we could pass it as a cli flag. With > > registration, we'd have to have some helper to get the KV ID. > > > > It also introduces ordering dependencies between the services on > > startup, eg packet tracing hooks could only be attached once our XDP > > service has registered a PACKET_ID KV, and they could query it's API. > > Yeah, we definitely need a way to make that accessible and not too > cumbersome. > > I suppose what we really need is a way to map an application-specific > identifier to an ID. And, well, those identifiers could just be (string) > names? That's what we do for CO-RE, after all. So you'd get something > like: > > id =3D bpf_register_metadata_field("packet_id", 8, BPF_CREATE); /* regist= er */ > > id =3D bpf_register_metadata_field("packet_id", 8, BPF_NO_CREATE); /* res= olve */ > > and we make that idempotent, so that two callers using the same name and > size will just get the same id back; and if called with BPF_NO_CREATE, > you'll get -ENOENT if it hasn't already been registered by someone else? > > We could even make this a sub-command of the bpf() syscall if we want it > to be UAPI, but that's not strictly necessary, applications can also > just call the registration from a syscall program at startup... That's a nice API, it makes sharing the IDs much easier. We still have to worry about collisions (what if two different things want to add their own "packet_id" field?). But at least: * "Any string" has many more possibilities than 0-64 keys. * bpf_register_metadata() could return an error if a field is already registered, instead of silently letting an application overwrite metadata (although arguably we could have add a BPF_NOEXIST style flag to the KV set() to kind of do the same). At least internally, it still feels like we'd maintain a registry of these string fields and make them configurable for each service to avoid collisions. > >> We could combine such a registration API with your header format, so > >> that the registration just becomes a way of allocating one of the keys > >> from 0-63 (and the registry just becomes a global copy of the header). > >> This would basically amount to moving the "service config file" into t= he > >> kernel, since that seems to be the only common denominator we can rely > >> on between BPF applications (as all attempts to write a common daemon > >> for BPF management have shown). > > > > That sounds reasonable. And I guess we'd have set() check the global > > registry to enforce that the key has been registered beforehand? > > Yes, exactly. And maybe check that the size matches as well just to > remove the obvious footgun of accidentally stepping on each other's > toes? > > > Thanks for all the feedback! > > You're welcome! Thanks for working on this :) > > -Toke