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[194.45.78.10]) by smtp.gmail.com with ESMTPSA id og40-20020a1709071de800b0095807ab4b57sm11264399ejc.178.2023.05.16.08.35.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 16 May 2023 08:35:29 -0700 (PDT) From: Jesper Dangaard Brouer X-Google-Original-From: Jesper Dangaard Brouer Message-ID: Date: Tue, 16 May 2023 17:35:27 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Cc: brouer@redhat.com, Jesper Dangaard Brouer , bpf@vger.kernel.org, Stanislav Fomichev , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Jakub Kicinski , Martin KaFai Lau , Song Liu , Yonghong Song , John Fastabend , KP Singh , Jiri Olsa , Jesse Brandeburg , Tony Nguyen , Anatoly Burakov , Alexander Lobakin , Magnus Karlsson , Maryam Tahhan , xdp-hints@xdp-project.net, netdev@vger.kernel.org, intel-wired-lan@lists.osuosl.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH RESEND bpf-next 14/15] net, xdp: allow metadata > 32 Content-Language: en-US To: Alexander Lobakin , Larysa Zaremba References: <20230512152607.992209-1-larysa.zaremba@intel.com> <20230512152607.992209-15-larysa.zaremba@intel.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 16/05/2023 14.37, Alexander Lobakin wrote: > From: Larysa Zaremba > Date: Mon, 15 May 2023 19:08:39 +0200 > >> On Mon, May 15, 2023 at 06:17:02PM +0200, Jesper Dangaard Brouer wrote: >>> >>> On 12/05/2023 17.26, Larysa Zaremba wrote: >>>> From: Aleksander Lobakin >>>> >>>> When using XDP hints, metadata sometimes has to be much bigger >>>> than 32 bytes. Relax the restriction, allow metadata larger than 32 bytes >>>> and make __skb_metadata_differs() work with bigger lengths. >>>> >>>> Now size of metadata is only limited by the fact it is stored as u8 >>>> in skb_shared_info, so maximum possible value is 255. >>> >>> I'm confused, IIRC the metadata area isn't stored "in skb_shared_info". >>> The maximum possible size is limited by the XDP headroom, which is also >>> shared/limited with/by xdp_frame. I must be reading the sentence wrong, >>> somehow. > > skb_shared_info::meta_size is u8. Since metadata gets carried from > xdp_buff to skb, this check is needed (it's compile-time constant anyway). > Check for headroom is done separately already (two sentences below). > Damn, argh, for SKBs the "meta_len" is stored in skb_shared_info, which is located on another cacheline. That is a sure way to KILL performance! :-( But only use for SKBs that gets created from xdp with metadata, right? >> It's not 'metadata is stored as u8', it's 'metadata size is stored as u8' :) >> Maybe I should rephrase it better in v2. Yes, a rephrase will be good. --Jesper static inline u8 skb_metadata_len(const struct sk_buff *skb) { return skb_shinfo(skb)->meta_len; }