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Miller" , Eric Dumazet , Paolo Abeni , Simon Horman , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , Stanislav Fomichev , Andrew Lunn , Tony Nguyen , Przemek Kitszel , Alexander Lobakin , Andrii Nakryiko , Martin KaFai Lau , Eduard Zingerman , Song Liu , Yonghong Song , KP Singh , Hao Luo , Jiri Olsa , Shuah Khan , Maciej Fijalkowski , Jonathan Corbet , Shuah Khan , Kumar Kartikeya Dwivedi , Emil Tsalapatis , Vladimir Vdovin , Jakub Sitnicki , netdev@vger.kernel.org, bpf@vger.kernel.org, intel-wired-lan@lists.osuosl.org, linux-kselftest@vger.kernel.org, linux-doc@vger.kernel.org Subject: Re: [PATCH bpf-next v5 8/8] selftests: net: add test for XDP_PASS skb checksum invalidation Message-ID: References: <20260715-bpf-xdp-meta-rxcksum-v5-0-623d5c0d0ab7@kernel.org> <20260715-bpf-xdp-meta-rxcksum-v5-8-623d5c0d0ab7@kernel.org> <20260721082728.74142e6c@kernel.org> Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20260721082728.74142e6c@kernel.org> On 07/21, Jakub Kicinski wrote: > On Mon, 20 Jul 2026 12:32:55 -0700 Stanislav Fomichev wrote: > > > > (although for gve I might be wrong, there is also gve_rx_skb_csum that only > > > > does UNNECESSARY). > > > > > > > > I'd wait for Jakub to chime in, but it feels like we should just document > > > > what we currently do as a recommended approach: for the drivers > > > > that support COMPLETE, do not report it when the bpf program is attached. > > > > Both NONE and UNNECESSARY are ok. > > > > > > I am not completely sure the UNNECESSARY case is different from the COMPLETE > > > one. What are we supposed to do if the driver reports UNNECESSARY and the ebpf > > > program modifies some fields covered by the rx-checksum? > > > > For unnecessary, I think the safe expectation is that the bpf program > > will update the value of the checksum in the packet if it touches the data? > > Documenting as expected behavior which no driver currently follows > is a bit silly. I thought the ask was to sketch out the plan of > explicitly updating/invalidating the checksum even if we don't > implement it today? This is about current drivers that only report UNNECESSARY with xdp: the xdp prog has to maintain in-packet checksum if it changes the payload. I think it's a fair assumption? In terms of documentation, here is what I have on my side, lmk if that makes sense, roughly: - TODAY - some drivers (correctly) disable reporting COMPLETE when XDP is attached - the xdp program has to modify the packet checksum value if it changes the payload - some drivers (incorrectly?) report COMPLETE for xdp-to-skb path -> unsafe, needs to be fixed - updating the payload doesn't update skb->csum, so the safest option right now is to only do UNNECESSARY with xdp for all drivers - the hw test needs to make sure that the csum is either NONE or UNNECESSARY, and will error out on COMPLETE - TOMORROW - if we want to properly support COMPLETE, we need a new set of kfuncs to update skb->csum value in the skb Does that make sense?