From patchwork Thu Dec 23 10:19:06 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Christoph Hellwig X-Patchwork-Id: 12698080 X-Patchwork-Delegate: bpf@iogearbox.net 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 B48A2C433EF for ; Thu, 23 Dec 2021 10:20:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1347671AbhLWKUb (ORCPT ); Thu, 23 Dec 2021 05:20:31 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46412 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1347682AbhLWKTf (ORCPT ); Thu, 23 Dec 2021 05:19:35 -0500 Received: from bombadil.infradead.org (bombadil.infradead.org [IPv6:2607:7c80:54:e::133]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DA087C061401; Thu, 23 Dec 2021 02:19:35 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20210309; h=Content-Transfer-Encoding: MIME-Version:References:In-Reply-To:Message-Id:Date:Subject:Cc:To:From:Sender :Reply-To:Content-Type:Content-ID:Content-Description; bh=YXA9bNbu70qCMUKMU1gqidDciHX65bz6vVCKVKz0r7I=; b=FVOVE7Nb0V3sHmy/eGPPRBgkp6 9IWtM+8HfpxA2qXkQWiH1oyRZYZMEuKIXxr8RaYxoZGH5R5h/iCD2npy6/rX0ILPwxn7ovuv9X8XA hw9TODB5mQeUpqr0sc0WZ+k1bJpoZZB7tdSMUlOWDsS+nimGFGdpp8SR29AcbzWrFAQs1N3Vrdtgz 6VD/KPLRhZ8pSr1gmy3tQx6JtTUrfXpUwGs4UYcL456PZh0rfj6f4A19ZvfmvpGN1+zveiXqwrhaO YyaCcRlWKvQsL3m+7zS4zq0iiQYxwA18KNu13hk9VnonGDBQAd+vsxVoxuTxAnOnv/Ji/fZcUdscS 8QfVjyQA==; Received: from [46.183.103.8] (helo=localhost) by bombadil.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1n0LC3-00CTmy-3G; Thu, 23 Dec 2021 10:19:27 +0000 From: Christoph Hellwig To: Jonathan Corbet , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko Cc: Martin KaFai Lau , Song Liu , Yonghong Song , John Fastabend , KP Singh , linux-doc@vger.kernel.org, netdev@vger.kernel.org, bpf@vger.kernel.org Subject: [PATCH 4/4] bpf, docs: Move the packet access instructions last in instruction-set.rst Date: Thu, 23 Dec 2021 11:19:06 +0100 Message-Id: <20211223101906.977624-5-hch@lst.de> X-Mailer: git-send-email 2.30.2 In-Reply-To: <20211223101906.977624-1-hch@lst.de> References: <20211223101906.977624-1-hch@lst.de> MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by bombadil.infradead.org. See http://www.infradead.org/rpr.html Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org X-Patchwork-Delegate: bpf@iogearbox.net The packet access instructions are a convoluted leftover from classic BPF. Move them last past the much more important atomic operations, and improve the rendering of the code example. Signed-off-by: Christoph Hellwig --- Documentation/bpf/instruction-set.rst | 55 +++++++++++++-------------- 1 file changed, 27 insertions(+), 28 deletions(-) diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst index 4e3041cf04325..922635f0c18b7 100644 --- a/Documentation/bpf/instruction-set.rst +++ b/Documentation/bpf/instruction-set.rst @@ -171,34 +171,6 @@ BPF_MEM | | BPF_LDX means:: Where size is one of: BPF_B or BPF_H or BPF_W or BPF_DW. -Packet access instructions --------------------------- - -eBPF has two non-generic instructions: (BPF_ABS | | BPF_LD) and -(BPF_IND | | BPF_LD) which are used to access packet data. - -They had to be carried over from classic BPF to have strong performance of -socket filters running in eBPF interpreter. These instructions can only -be used when interpreter context is a pointer to ``struct sk_buff`` and -have seven implicit operands. Register R6 is an implicit input that must -contain pointer to sk_buff. Register R0 is an implicit output which contains -the data fetched from the packet. Registers R1-R5 are scratch registers -and must not be used to store the data across BPF_ABS | BPF_LD or -BPF_IND | BPF_LD instructions. - -These instructions have implicit program exit condition as well. When -eBPF program is trying to access the data beyond the packet boundary, -the interpreter will abort the execution of the program. JIT compilers -therefore must preserve this property. src_reg and imm32 fields are -explicit inputs to these instructions. - -For example:: - - BPF_IND | BPF_W | BPF_LD means: - - R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + src_reg + imm32)) - and R1 - R5 were scratched. - Atomic operations ----------------- @@ -252,3 +224,30 @@ zero. eBPF has one 16-byte instruction: ``BPF_LD | BPF_DW | BPF_IMM`` which consists of two consecutive ``struct bpf_insn`` 8-byte blocks and interpreted as single instruction that loads 64-bit immediate value into a dst_reg. + +Packet access instructions +-------------------------- + +eBPF has two non-generic instructions: (BPF_ABS | | BPF_LD) and +(BPF_IND | | BPF_LD) which are used to access packet data. + +They had to be carried over from classic BPF to have strong performance of +socket filters running in eBPF interpreter. These instructions can only +be used when interpreter context is a pointer to ``struct sk_buff`` and +have seven implicit operands. Register R6 is an implicit input that must +contain pointer to sk_buff. Register R0 is an implicit output which contains +the data fetched from the packet. Registers R1-R5 are scratch registers +and must not be used to store the data across BPF_ABS | BPF_LD or +BPF_IND | BPF_LD instructions. + +These instructions have implicit program exit condition as well. When +eBPF program is trying to access the data beyond the packet boundary, +the interpreter will abort the execution of the program. JIT compilers +therefore must preserve this property. src_reg and imm32 fields are +explicit inputs to these instructions. + +For example, BPF_IND | BPF_W | BPF_LD means:: + + R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + src_reg + imm32)) + +and R1 - R5 are clobbered.