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Fri, 30 Jun 2023 02:19:42 -0700 (PDT) X-Google-Smtp-Source: APBJJlEiu9qdB6QGUeX35Ichhu5tRhcOShyMGUgce8+gbAXPir+BD7ANw0AC1LrPhlxF8aSbxLZKaw== X-Received: by 2002:a17:907:2157:b0:991:c566:979 with SMTP id rk23-20020a170907215700b00991c5660979mr1388779ejb.36.1688116781769; Fri, 30 Jun 2023 02:19:41 -0700 (PDT) Received: from alrua-x1.borgediget.toke.dk ([2a0c:4d80:42:443::2]) by smtp.gmail.com with ESMTPSA id lr9-20020a170906fb8900b00969f44bbef3sm7949007ejb.11.2023.06.30.02.19.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 30 Jun 2023 02:19:41 -0700 (PDT) Received: by alrua-x1.borgediget.toke.dk (Postfix, from userid 1000) id 98402BC06D5; Fri, 30 Jun 2023 11:19:40 +0200 (CEST) From: Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= To: Magnus Karlsson In-Reply-To: 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> <871qhuh5ec.fsf@toke.dk> <87r0pufpf2.fsf@toke.dk> X-Clacks-Overhead: GNU Terry Pratchett Date: Fri, 30 Jun 2023 11:19:40 +0200 Message-ID: <87pm5de28j.fsf@toke.dk> MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Message-ID-Hash: OYHY6XPCWBO6XYYXK6JZF2GKXKIO3XLZ X-Message-ID-Hash: OYHY6XPCWBO6XYYXK6JZF2GKXKIO3XLZ X-MailFrom: toke@redhat.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: Stanislav Fomichev , 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: Magnus Karlsson writes: > On Thu, 29 Jun 2023 at 14:01, Toke H=C3=B8iland-J=C3=B8rgensen wrote: >> >> Magnus Karlsson writes: >> >> > On Thu, 29 Jun 2023 at 13:30, Toke H=C3=B8iland-J=C3=B8rgensen wrote: >> >> >> >> Stanislav Fomichev writes: >> >> >> >> > 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 Brou= er >> >> >> > > > wrote: >> >> >> > > >> >> >> >> > > >> >> >> >> > > >> >> >> >> > > >> On 22/06/2023 19.55, Stanislav Fomichev wrote: >> >> >> > > >>> On Thu, Jun 22, 2023 at 2:11=E2=80=AFAM Jesper D. Brouer <= netdev@brouer.com> wrote: >> >> >> > > >>>> >> >> >> > > >>>> >> >> >> > > >>>> This needs to be reviewed by AF_XDP maintainers Magnus an= d Bj=C3=B8rn (Cc) >> >> >> > > >>>> >> >> >> > > >>>> On 21/06/2023 19.02, Stanislav Fomichev wrote: >> >> >> > > >>>>> For zerocopy mode, tx_desc->addr can point to the arbitr= ary offset >> >> >> > > >>>>> and carry some TX metadata in the headroom. For copy mod= e, there >> >> >> > > >>>>> is no way currently to populate skb metadata. >> >> >> > > >>>>> >> >> >> > > >>>>> Introduce new XDP_TX_METADATA_LEN that indicates how man= y bytes >> >> >> > > >>>>> to treat as metadata. Metadata bytes come prior to tx_de= sc address >> >> >> > > >>>>> (same as in RX case). >> >> >> > > >>>> >> >> >> > > >>>> From looking at the code, this introduces a socket opt= ion for this TX >> >> >> > > >>>> metadata length (tx_metadata_len). >> >> >> > > >>>> This implies the same fixed TX metadata size is used for = all packets. >> >> >> > > >>>> Maybe describe this in patch desc. >> >> >> > > >>> >> >> >> > > >>> I was planning to do a proper documentation page once we s= ettle on all >> >> >> > > >>> the details (similar to the one we have for rx). >> >> >> > > >>> >> >> >> > > >>>> What is the plan for dealing with cases that doesn't popu= late same/full >> >> >> > > >>>> TX metadata size ? >> >> >> > > >>> >> >> >> > > >>> Do we need to support that? I was assuming that the TX lay= out would be >> >> >> > > >>> fixed between the userspace and BPF. >> >> >> > > >> >> >> >> > > >> I hope you don't mean fixed layout, as the whole point is a= dding >> >> >> > > >> flexibility and extensibility. >> >> >> > > > >> >> >> > > > I do mean a fixed layout between the userspace (af_xdp) and = devtx program. >> >> >> > > > At least fixed max size of the metadata. The userspace and t= he bpf >> >> >> > > > prog can then use this fixed space to implement some flexibi= lity >> >> >> > > > (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/li= mited >> >> >> > > sized memory area, but communicate size per packet in this are= a (also >> >> >> > > for validation purposes). BUT for AF_XDP we don't have room f= or another >> >> >> > > 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 cast in >> >> >> > > >> BPF-prog. Both BPF-prog and userspace have access to the lo= cal BTF ids, >> >> >> > > >> see [1] and [2]. >> >> >> > > >> >> >> >> > > >> It seems we are talking slightly past each-other(?). Let m= e rephrase >> >> >> > > >> and reframe the question, what is your *plan* for dealing w= ith different >> >> >> > > >> *types* of TX metadata. The different struct *types* will = of-cause have >> >> >> > > >> different sizes, but that is okay as long as they fit into = the maximum >> >> >> > > >> size set by this new socket option XDP_TX_METADATA_LEN. >> >> >> > > >> Thus, in principle I'm fine with XSK having configured a fi= xed headroom >> >> >> > > >> for metadata, but we need a plan for handling more than one= type and >> >> >> > > >> perhaps a xsk desc indicator/flag for knowing TX metadata i= sn't random >> >> >> > > >> data ("leftover" since last time this mem was used). >> >> >> > > > >> >> >> > > > Yeah, I think the above correctly catches my expectation her= e. Some >> >> >> > > > headroom is reserved via XDP_TX_METADATA_LEN and the flexibi= lity is >> >> >> > > > offloaded to the bpf program via btf_id/tlv/etc. >> >> >> > > > >> >> >> > > > Regarding leftover metadata: can we assume the userspace wil= l take >> >> >> > > > care of setting it up? >> >> >> > > > >> >> >> > > >> With this kfunc approach, then things in-principle, becomes= a contract >> >> >> > > >> between the "local" TX-hook BPF-prog and AF_XDP userspace. = These two >> >> >> > > >> components can as illustrated here [1]+[2] can coordinate b= ased on local >> >> >> > > >> BPF-prog BTF IDs. This approach works as-is today, but pat= chset >> >> >> > > >> selftests examples don't use this and instead have a very s= tatic >> >> >> > > >> approach (that people will copy-paste). >> >> >> > > >> >> >> >> > > >> An unsolved problem with TX-hook is that it can also get pa= ckets from >> >> >> > > >> XDP_REDIRECT and even normal SKBs gets processed (right?). = How does 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 filte= ring. Plus >> >> >> > > > 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 comm= unicate >> >> >> > > activation of TX hardware hints is too weak and not enough. T= his is an >> >> >> > > implicit API for BPF-programmers to understand and can lead to= implicit >> >> >> > > activation. >> >> >> > > >> >> >> > > Think about what will happen for your AF_XDP send use-case. F= or >> >> >> > > performance reasons AF_XDP don't zero out frame memory. Thus,= meta_len >> >> >> > > is fixed even if not used (and can contain garbage), it can by= accident >> >> >> > > create hard-to-debug situations. As discussed with Magnus+Mar= yam >> >> >> > > before, we found it was practical (and faster than mem zero) t= o extend >> >> >> > > 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 ha= ve >> >> >> > is_used bit? The userspace has to initialize at least that bit. >> >> >> > We can definitely add that extra "has_metadata" bit to the descr= iptor, >> >> >> > 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 th= e >> >> >> metadata field and you save some performance. Regardless of this b= it, >> >> >> you need some way to say "is_used" for each metadata entry (at lea= st >> >> >> 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 fo= r >> >> >> every metadata and requires the user/kernel to go scan through eve= ry >> >> >> entry for every packet. >> >> >> >> >> >> #2: Have a field of bits at the start of the metadata section (clo= sest >> >> >> 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 us= ed >> >> >> or not. Only requires the user/kernel to scan the valid entries pl= us >> >> >> 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 metada= ta >> >> >> 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 dari= ng >> >> >> 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 itsel= f. >> >> >> 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 fo= r >> >> >> the metadata) should be the fastest. The decision also depends on = the >> >> >> number of metadata entries you have per packet. Sorry that I do no= t >> >> >> have a good answer. My feeling is that we need something like #1 o= r >> >> >> #2, or maybe both, then if needed we can add the "has_metadata" bi= t 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 the= y >> >> > 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 someh= ow >> >> > (devtx_frame.meta_len in this series). >> >> > Then it's up to the userspace and bpf program to agree on the layou= t. >> >> > 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 p= rog >> >> > to interpret it and call appropriate kfunc to enable some offload. >> >> >> >> The reason for the flag on RX is mostly performance: there's a >> >> substantial performance hit from reading the metadata area because it= 's >> >> not cache-hot; we want to avoid that when no metadata is in use. Putt= ing >> >> the flag inside the metadata area itself doesn't work for this, becau= se >> >> then you incur the cache miss just to read the flag. >> > >> > Not necessarily. Let us say that the flag is 4 bytes. Increase the >> > start address of the packet buffer with 4 and the flags field will be >> > on the same cache line as the first 60 bytes of the packet data >> > (assuming a 64 byte cache line size and the flags field is closest to >> > the start of the packet data). As long as you write something in those >> > first 60 bytes of packet data that cache line will be brought in and >> > will likely be in the cache when you access the bits in the metadata >> > field. The trick works similarly for Rx by setting the umem headroom >> > accordingly. >> >> Yeah, a trick like that was what I was alluding to with the "could" in >> this bit: >> >> >> but I see no reason it could not also occur on TX (it'll mostly >> >> depend on data alignment I guess?). >> >> right below the text you quoted ;) > > Ouch! Sorry Toke. Was a bit too trigger-happy there. Haha, no worries, seems like we're basically in agreement anyway :) -Toke