From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pg1-x536.google.com (mail-pg1-x536.google.com [IPv6:2607:f8b0:4864:20::536]) by mail.toke.dk (Postfix) with ESMTPS id 23AE5A13810 for ; Wed, 28 Jun 2023 20:49:41 +0200 (CEST) Authentication-Results: mail.toke.dk; dkim=pass (2048-bit key; unprotected) header.d=google.com header.i=@google.com header.a=rsa-sha256 header.s=20221208 header.b=bFxxvi0N Received: by mail-pg1-x536.google.com with SMTP id 41be03b00d2f7-52cb8e5e9f5so162727a12.0 for ; Wed, 28 Jun 2023 11:49:41 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20221208; t=1687978178; x=1690570178; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:from:to:cc:subject:date :message-id:reply-to; bh=KgaN56lEm0ZmpqNHWkSOtd80x9mkubEd8uoaFEqBMgs=; b=bFxxvi0NVv6rMtyYu10FMUz8qfbPjlLsR52jO6dscEReVBu0cHa9NHSVZMYpiAl3hL uULfGznVRWlMq/329brqNqXHhsOyoKW+ro8eFEUHIU11Z/BwJ2mhxUjbKrzBJKXnaqeX VEMhwUczUiIwvDohMnwisAtbxVRJYIrkLlMV7/tbK9Y/wwxIHZBo/JlLTYk2bD2kUH4l CEUVPt7uKuFBekT/NewzmPw/zef2YPscEElJqst9ZADiFV0zEPuFKYWGesiC3ht1DEkf Lj6PWndVmVfilwHcCL2CtrBq5smyWForJhp2boT4GqfUDM67j0HSAeVoZsvPm94jH4NO DPBw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1687978178; x=1690570178; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=KgaN56lEm0ZmpqNHWkSOtd80x9mkubEd8uoaFEqBMgs=; b=BpRIQbvylZk7bI8cTTIGY7hOzEbLq+qN7hUtUEh0yh1TU/mtfNart+7D2k2Af+NfaS 5y4inXtaOsI5ZllVHWN45wqlfvp8QOJdsUDP7idJtT8kCGBj2Fbg5JhNQCcMNxS+EQO8 FtCQBtrO3tn44zSS5gsUl/OR7DzaqB4yaaFYyyR6MCozHQq4UzE3Zm/n/CLB6gFtxQK7 g4OYXvvSi9xMt//LoLPHrtUGygd87/Viph64J3T/VCx41asrcAaitsbsoToguxnRb4du HNyse21rJPQfHLrYDMoDGFZJMcFR1F85znJQRf/TrwDFY+F3fqz0ZWGDypO3Uf9CxA07 BGCw== X-Gm-Message-State: AC+VfDz9Angv+LHcr431UdftmL223pjMWRUXSvmgI844qynoIzpD6vy1 NoNb3eifGRb7O/jFbPZ6dX4OMOIdp7En+210gF5Cqg== X-Google-Smtp-Source: ACHHUZ4IbfKMVHThMuQt9NsFrggAFQx4Rv5qpBGTG3h+eAy6ncxooyhJNA3gaLLBdOgPONwb5i0/LZYb5dES96URox4= X-Received: by 2002:a17:90b:2d8d:b0:262:fe4b:b45 with SMTP id sj13-20020a17090b2d8d00b00262fe4b0b45mr2600575pjb.19.1687978178329; Wed, 28 Jun 2023 11:49:38 -0700 (PDT) MIME-Version: 1.0 References: <20230621170244.1283336-1-sdf@google.com> <20230621170244.1283336-4-sdf@google.com> <57b9fc14-c02e-f0e5-148d-a549ebab6cf6@brouer.com> <435d1630-c3f4-97fb-b6fe-9795d1f0bf33@redhat.com> In-Reply-To: From: Stanislav Fomichev Date: Wed, 28 Jun 2023 11:49:25 -0700 Message-ID: To: Magnus Karlsson Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Message-ID-Hash: MGZKUJAU2Z6L2GAT4Y4CVTWD7WZHCY7I X-Message-ID-Hash: MGZKUJAU2Z6L2GAT4Y4CVTWD7WZHCY7I X-MailFrom: sdf@google.com X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: Jesper Dangaard Brouer , brouer@redhat.com, bpf@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev, song@kernel.org, yhs@fb.com, john.fastabend@gmail.com, kpsingh@kernel.org, haoluo@google.com, jolsa@kernel.org, =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= , "Karlsson, Magnus" , "xdp-hints@xdp-project.net" X-Mailman-Version: 3.3.8 Precedence: list Subject: [xdp-hints] Re: [RFC bpf-next v2 03/11] xsk: Support XDP_TX_METADATA_LEN List-Id: XDP hardware hints design discussion Archived-At: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: On Wed, Jun 28, 2023 at 1:09=E2=80=AFAM Magnus Karlsson wrote: > > On Mon, 26 Jun 2023 at 19:06, Stanislav Fomichev wrote: > > > > On Sat, Jun 24, 2023 at 2:02=E2=80=AFAM Jesper Dangaard Brouer > > wrote: > > > > > > > > > > > > On 23/06/2023 19.41, Stanislav Fomichev wrote: > > > > On Fri, Jun 23, 2023 at 3:24=E2=80=AFAM Jesper Dangaard Brouer > > > > wrote: > > > >> > > > >> > > > >> > > > >> On 22/06/2023 19.55, Stanislav Fomichev wrote: > > > >>> On Thu, Jun 22, 2023 at 2:11=E2=80=AFAM Jesper D. Brouer wrote: > > > >>>> > > > >>>> > > > >>>> This needs to be reviewed by AF_XDP maintainers Magnus and Bj=C3= =B8rn (Cc) > > > >>>> > > > >>>> On 21/06/2023 19.02, Stanislav Fomichev wrote: > > > >>>>> For zerocopy mode, tx_desc->addr can point to the arbitrary off= set > > > >>>>> and carry some TX metadata in the headroom. For copy mode, ther= e > > > >>>>> is no way currently to populate skb metadata. > > > >>>>> > > > >>>>> Introduce new XDP_TX_METADATA_LEN that indicates how many bytes > > > >>>>> to treat as metadata. Metadata bytes come prior to tx_desc addr= ess > > > >>>>> (same as in RX case). > > > >>>> > > > >>>> From looking at the code, this introduces a socket option for= this TX > > > >>>> metadata length (tx_metadata_len). > > > >>>> This implies the same fixed TX metadata size is used for all pac= kets. > > > >>>> Maybe describe this in patch desc. > > > >>> > > > >>> I was planning to do a proper documentation page once we settle o= n all > > > >>> the details (similar to the one we have for rx). > > > >>> > > > >>>> What is the plan for dealing with cases that doesn't populate sa= me/full > > > >>>> TX metadata size ? > > > >>> > > > >>> Do we need to support that? I was assuming that the TX layout wou= ld be > > > >>> fixed between the userspace and BPF. > > > >> > > > >> I hope you don't mean fixed layout, as the whole point is adding > > > >> flexibility and extensibility. > > > > > > > > I do mean a fixed layout between the userspace (af_xdp) and devtx p= rogram. > > > > At least fixed max size of the metadata. The userspace and the bpf > > > > prog can then use this fixed space to implement some flexibility > > > > (btf_ids, versioned structs, bitmasks, tlv, etc). > > > > If we were to make the metalen vary per packet, we'd have to signal > > > > its size per packet. Probably not worth it? > > > > > > Existing XDP metadata implementation also expand in a fixed/limited > > > sized memory area, but communicate size per packet in this area (also > > > for validation purposes). BUT for AF_XDP we don't have room for anot= her > > > pointer or size in the AF_XDP descriptor (see struct xdp_desc). > > > > > > > > > > > > > >>> If every packet would have a different metadata length, it seems = like > > > >>> a nightmare to parse? > > > >>> > > > >> > > > >> No parsing is really needed. We can simply use BTF IDs and type c= ast in > > > >> BPF-prog. Both BPF-prog and userspace have access to the local BTF= ids, > > > >> see [1] and [2]. > > > >> > > > >> It seems we are talking slightly past each-other(?). Let me rephr= ase > > > >> and reframe the question, what is your *plan* for dealing with dif= ferent > > > >> *types* of TX metadata. The different struct *types* will of-caus= e have > > > >> different sizes, but that is okay as long as they fit into the max= imum > > > >> size set by this new socket option XDP_TX_METADATA_LEN. > > > >> Thus, in principle I'm fine with XSK having configured a fixed hea= droom > > > >> for metadata, but we need a plan for handling more than one type a= nd > > > >> perhaps a xsk desc indicator/flag for knowing TX metadata isn't ra= ndom > > > >> data ("leftover" since last time this mem was used). > > > > > > > > Yeah, I think the above correctly catches my expectation here. Some > > > > headroom is reserved via XDP_TX_METADATA_LEN and the flexibility is > > > > offloaded to the bpf program via btf_id/tlv/etc. > > > > > > > > Regarding leftover metadata: can we assume the userspace will take > > > > care of setting it up? > > > > > > > >> With this kfunc approach, then things in-principle, becomes a cont= ract > > > >> between the "local" TX-hook BPF-prog and AF_XDP userspace. These= two > > > >> components can as illustrated here [1]+[2] can coordinate based on= local > > > >> BPF-prog BTF IDs. This approach works as-is today, but patchset > > > >> selftests examples don't use this and instead have a very static > > > >> approach (that people will copy-paste). > > > >> > > > >> An unsolved problem with TX-hook is that it can also get packets f= rom > > > >> XDP_REDIRECT and even normal SKBs gets processed (right?). How do= es the > > > >> BPF-prog know if metadata is valid and intended to be used for e.g= . > > > >> requesting the timestamp? (imagine metadata size happen to match) > > > > > > > > My assumption was the bpf program can do ifindex/netns filtering. P= lus > > > > maybe check that the meta_len is the one that's expected. > > > > Will that be enough to handle XDP_REDIRECT? > > > > > > I don't think so, using the meta_len (+ ifindex/netns) to communicate > > > activation of TX hardware hints is too weak and not enough. This is = an > > > implicit API for BPF-programmers to understand and can lead to implic= it > > > activation. > > > > > > Think about what will happen for your AF_XDP send use-case. For > > > performance reasons AF_XDP don't zero out frame memory. Thus, meta_l= en > > > is fixed even if not used (and can contain garbage), it can by accide= nt > > > create hard-to-debug situations. As discussed with Magnus+Maryam > > > before, we found it was practical (and faster than mem zero) to exten= d > > > AF_XDP descriptor (see struct xdp_desc) with some flags to > > > indicate/communicate this frame comes with TX metadata hints. > > > > What is that "if not used" situation? Can the metadata itself have > > is_used bit? The userspace has to initialize at least that bit. > > We can definitely add that extra "has_metadata" bit to the descriptor, > > but I'm trying to understand whether we can do without it. > > To me, this "has_metadata" bit in the descriptor is just an > optimization. If it is 0, then there is no need to go and check the > metadata field and you save some performance. Regardless of this bit, > you need some way to say "is_used" for each metadata entry (at least > when the number of metadata entries is >1). Three options come to mind > each with their pros and cons. > > #1: Let each metadata entry have an invalid state. Not possible for > every metadata and requires the user/kernel to go scan through every > entry for every packet. > > #2: Have a field of bits at the start of the metadata section (closest > to packet data) that signifies if a metadata entry is valid or not. If > there are N metadata entries in the metadata area, then N bits in this > field would be used to signify if the corresponding metadata is used > or not. Only requires the user/kernel to scan the valid entries plus > one access for the "is_used" bits. > > #3: Have N bits in the AF_XDP descriptor options field instead of the > N bits in the metadata area of #2. Faster but would consume many > precious bits in the fixed descriptor and cap the number of metadata > entries possible at around 8. E.g., 8 for Rx, 8 for Tx, 1 for the > multi-buffer work, and 15 for some future use. Depends on how daring > we are. > > The "has_metadata" bit suggestion can be combined with 1 or 2. > Approach 3 is just a fine grained extension of the idea itself. > > IMO, the best approach unfortunately depends on the metadata itself. > If it is rarely valid, you want something like the "has_metadata" bit. > If it is nearly always valid and used, approach #1 (if possible for > the metadata) should be the fastest. The decision also depends on the > number of metadata entries you have per packet. Sorry that I do not > have a good answer. My feeling is that we need something like #1 or > #2, or maybe both, then if needed we can add the "has_metadata" bit or > bits (#3) optimization. Can we do this encoding and choice (#1, #2, or > a combo) in the eBPF program itself? Would provide us with the > flexibility, if possible. Here is my take on it, lmk if I'm missing something: af_xdp users call this new setsockopt(XDP_TX_METADATA_LEN) when they plan to use metadata on tx. This essentially requires allocating a fixed headroom to carry the metadata= . af_xdp machinery exports this fixed len into the bpf programs somehow (devtx_frame.meta_len in this series). Then it's up to the userspace and bpf program to agree on the layout. If not every packet is expected to carry the metadata, there might be some bitmask in the metadata area to indicate that. Iow, the metadata isn't interpreted by the kernel. It's up to the prog to interpret it and call appropriate kfunc to enable some offload. Jesper raises a valid point with "what about redirected packets?". But I'm not sure we need to care? Presumably the programs that do xdp_redirect will have to conform to the same metadata layout?