From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 71D62C433FE for ; Tue, 15 Nov 2022 22:44:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229637AbiKOWox (ORCPT ); Tue, 15 Nov 2022 17:44:53 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42038 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231222AbiKOWoK (ORCPT ); Tue, 15 Nov 2022 17:44:10 -0500 Received: from mail-il1-x12d.google.com (mail-il1-x12d.google.com [IPv6:2607:f8b0:4864:20::12d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E4EA718B0A for ; Tue, 15 Nov 2022 14:44:09 -0800 (PST) Received: by mail-il1-x12d.google.com with SMTP id q5so8211802ilt.13 for ; Tue, 15 Nov 2022 14:44:09 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date:message-id:reply-to; bh=1x2/L1T0WtWyFMgFLrb66QK4cplfsf+uyqGkhNI2KKQ=; b=WFgNUnl4chiFWm5ZNrOCo1LqKDET2aBPgVQ7Dm3+QxxdR1uM453Am+3Mawo2VLXCjG 75zt8NOvfE7Uov2R2+dqzQfHvcjw1QocG0sMScVyTwQ7spC4clR5xzdhCiMnschCmXgt LYHYn5l9U9LXC91l5brUL80G3ZuvNai97jXkKVl1kiqpH9w/dvhnLEcQIDiwpDtvLyzD pun7P5k0esM8BSIm78oGv+vjgdIfGVkcJtiSzfDJfDI4OM9W4KIyPkT9cPcLX/lkdDqk esUPZLpZf7N/DLA6eOfxZaR+GCD2EBXlN/ii84TtZB1NVZlecKxfDbv0ISwcjbbXEPQz qFag== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=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=1x2/L1T0WtWyFMgFLrb66QK4cplfsf+uyqGkhNI2KKQ=; b=pJB7BchbGK/XNFj09ISUG19YT7U8c2pt9+7Fa8VnIgDozUuyeBrAMlNfVr7roQzUyD gLrm92irOBuh62KHibM6anOWKw+t16fw/c/L9VmLSNE9U3h9BW08YOdBDF7eP85Iq+bf 9BMnBZZ5fZcie7y921B1sufkdRsMdpYbD4RyC7j7fTpAEKO+rv0NsizTdjPis49G+QPa +4CF8jW2YHu1QUo/M2+TlKV2ScoJKgFZ60z3fx1eN6OSC7lXVfPdKUR7sMEPJSQj1kSO w3eq+xOIORMzGKVmaznUWn7/UEF0UnJUcQcITjFyqKSVahoEpshkeEPsbU/vf9nxLzat TH9w== X-Gm-Message-State: ANoB5plp7S8VSyuAaE8TdHlnCYCxY+Wn6NeZ+2AGq+nZ+HCE9MbJ3JxY vvy9IFdLNoHz8roqnOSOx3bTPd0HYVV2dX0BBDwZquLrzQ+J9Q== X-Google-Smtp-Source: AA0mqf7GELw7CYrBJ/6SNPTe+5yS75odkqth7YIn/VDPyYoTVwZsIkMpI143fft/c3kMJ8Nz7qIBbEf9CwVMkothVKw= X-Received: by 2002:a92:d1c4:0:b0:2ea:ba31:f2ef with SMTP id u4-20020a92d1c4000000b002eaba31f2efmr9137492ilg.159.1668552249098; Tue, 15 Nov 2022 14:44:09 -0800 (PST) MIME-Version: 1.0 References: <20221115030210.3159213-1-sdf@google.com> <20221115030210.3159213-2-sdf@google.com> In-Reply-To: From: Stanislav Fomichev Date: Tue, 15 Nov 2022 14:43:58 -0800 Message-ID: Subject: Re: [PATCH bpf-next 01/11] bpf: Document XDP RX metadata To: Zvi Effron Cc: 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 Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Tue, Nov 15, 2022 at 2:31 PM Zvi Effron wrote: > > On Mon, Nov 14, 2022 at 7:04 PM Stanislav Fomichev wrote: > > > > Document all current use-cases and assumptions. > > > > Signed-off-by: Stanislav Fomichev > > --- > > Documentation/bpf/xdp-rx-metadata.rst | 109 ++++++++++++++++++++++++++ > > 1 file changed, 109 insertions(+) > > create mode 100644 Documentation/bpf/xdp-rx-metadata.rst > > > > diff --git a/Documentation/bpf/xdp-rx-metadata.rst b/Documentation/bpf/xdp-rx-metadata.rst > > new file mode 100644 > > index 000000000000..5ddaaab8de31 > > --- /dev/null > > +++ b/Documentation/bpf/xdp-rx-metadata.rst > > @@ -0,0 +1,109 @@ > > +=============== > > +XDP RX Metadata > > +=============== > > + > > +XDP programs support creating and passing custom metadata via > > +``bpf_xdp_adjust_meta``. This metadata can be consumed by the following > > +entities: > > + > > +1. ``AF_XDP`` consumer. > > +2. Kernel core stack via ``XDP_PASS``. > > +3. Another device via ``bpf_redirect_map``. > > 4. Other eBPF programs via eBPF tail calls. Don't think a tail call is a special case here? Correct me if I'm wrong, but with a tail call, we retain the original xdp_buff ctx, so the tail call can still use the same kfuncs as if the original bpf prog was running. > > + > > +General Design > > +============== > > + > > +XDP has access to a set of kfuncs to manipulate the metadata. Every > > +device driver implements these kfuncs by generating BPF bytecode > > +to parse it out from the hardware descriptors. The set of kfuncs is > > +declared in ``include/net/xdp.h`` via ``XDP_METADATA_KFUNC_xxx``. > > + > > +Currently, the following kfuncs are supported. In the future, as more > > +metadata is supported, this set will grow: > > + > > +- ``bpf_xdp_metadata_rx_timestamp_supported`` returns true/false to > > + indicate whether the device supports RX timestamps in general > > +- ``bpf_xdp_metadata_rx_timestamp`` returns packet RX timestamp or 0 > > +- ``bpf_xdp_metadata_export_to_skb`` prepares metadata layout that > > + the kernel will be able to consume. See ``bpf_redirect_map`` section > > + below for more details. > > + > > +Within the XDP frame, the metadata layout is as follows:: > > + > > + +----------+------------------+-----------------+------+ > > + | headroom | xdp_skb_metadata | custom metadata | data | > > + +----------+------------------+-----------------+------+ > > + ^ ^ > > + | | > > + xdp_buff->data_meta xdp_buff->data > > + > > +Where ``xdp_skb_metadata`` is the metadata prepared by > > +``bpf_xdp_metadata_export_to_skb``. And ``custom metadata`` > > +is prepared by the BPF program via calls to ``bpf_xdp_adjust_meta``. > > + > > +Note that ``bpf_xdp_metadata_export_to_skb`` doesn't adjust > > +``xdp->data_meta`` pointer. To access the metadata generated > > +by ``bpf_xdp_metadata_export_to_skb`` use ``xdp_buf->skb_metadata``. > > + > > +AF_XDP > > +====== > > + > > +``AF_XDP`` use-case implies that there is a contract between the BPF program > > +that redirects XDP frames into the ``XSK`` and the final consumer. > > +Thus the BPF program manually allocates a fixed number of > > +bytes out of metadata via ``bpf_xdp_adjust_meta`` and calls a subset > > +of kfuncs to populate it. User-space ``XSK`` consumer, looks > > +at ``xsk_umem__get_data() - METADATA_SIZE`` to locate its metadata. > > + > > +Here is the ``AF_XDP`` consumer layout (note missing ``data_meta`` pointer):: > > + > > + +----------+------------------+-----------------+------+ > > + | headroom | xdp_skb_metadata | custom metadata | data | > > + +----------+------------------+-----------------+------+ > > + ^ > > + | > > + rx_desc->address > > + > > +XDP_PASS > > +======== > > + > > +This is the path where the packets processed by the XDP program are passed > > +into the kernel. The kernel creates ``skb`` out of the ``xdp_buff`` contents. > > +Currently, every driver has a custom kernel code to parse the descriptors and > > +populate ``skb`` metadata when doing this ``xdp_buff->skb`` conversion. > > +In the future, we'd like to support a case where XDP program can override > > +some of that metadata. > > + > > +The plan of record is to make this path similar to ``bpf_redirect_map`` > > +below where the program would call ``bpf_xdp_metadata_export_to_skb``, > > +override the metadata and return ``XDP_PASS``. Additional work in > > +the drivers will be required to enable this (for example, to skip > > +populating ``skb`` metadata from the descriptors when > > +``bpf_xdp_metadata_export_to_skb`` has been called). > > + > > +bpf_redirect_map > > +================ > > + > > +``bpf_redirect_map`` can redirect the frame to a different device. > > +In this case we don't know ahead of time whether that final consumer > > +will further redirect to an ``XSK`` or pass it to the kernel via ``XDP_PASS``. > > +Additionally, the final consumer doesn't have access to the original > > +hardware descriptor and can't access any of the original metadata. > > + > > +To support passing metadata via ``bpf_redirect_map``, there is a > > +``bpf_xdp_metadata_export_to_skb`` kfunc that populates a subset > > +of metadata into ``xdp_buff``. The layout is defined in > > +``struct xdp_skb_metadata``. > > + > > +Mixing custom metadata and xdp_skb_metadata > > +=========================================== > > + > > +For the cases of ``bpf_redirect_map``, where the final consumer isn't > > +known ahead of time, the program can store both, custom metadata > > +and ``xdp_skb_metadata`` for the kernel consumption. > > + > > +Current limitation is that the program cannot adjust ``data_meta`` (via > > +``bpf_xdp_adjust_meta``) after a call to ``bpf_xdp_metadata_export_to_skb``. > > +So it has to, first, prepare its custom metadata layout and only then, > > +optionally, store ``xdp_skb_metadata`` via a call to > > +``bpf_xdp_metadata_export_to_skb``. > > -- > > 2.38.1.431.g37b22c650d-goog > >