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 X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 03101C47404 for ; Fri, 11 Oct 2019 21:26:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C921A214E0 for ; Fri, 11 Oct 2019 21:26:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729140AbfJKV0g (ORCPT ); Fri, 11 Oct 2019 17:26:36 -0400 Received: from www62.your-server.de ([213.133.104.62]:49500 "EHLO www62.your-server.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726255AbfJKV0g (ORCPT ); Fri, 11 Oct 2019 17:26:36 -0400 Received: from 55.249.197.178.dynamic.dsl-lte-bonding.lssmb00p-msn.res.cust.swisscom.ch ([178.197.249.55] helo=localhost) by www62.your-server.de with esmtpsa (TLSv1.2:DHE-RSA-AES256-GCM-SHA384:256) (Exim 4.89_1) (envelope-from ) id 1iJ2Qh-00078r-Oh; Fri, 11 Oct 2019 23:26:31 +0200 Date: Fri, 11 Oct 2019 23:26:31 +0200 From: Daniel Borkmann To: Andrii Nakryiko Cc: bpf@vger.kernel.org, netdev@vger.kernel.org, ast@fb.com, andrii.nakryiko@gmail.com, kernel-team@fb.com Subject: Re: [PATCH bpf-next] libbpf: generate more efficient BPF_CORE_READ code Message-ID: <20191011212631.GE21131@pc-63.home> References: <20191011023847.275936-1-andriin@fb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191011023847.275936-1-andriin@fb.com> User-Agent: Mutt/1.12.1 (2019-06-15) X-Authenticated-Sender: daniel@iogearbox.net X-Virus-Scanned: Clear (ClamAV 0.101.4/25599/Fri Oct 11 10:48:23 2019) Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Thu, Oct 10, 2019 at 07:38:47PM -0700, Andrii Nakryiko wrote: > Existing BPF_CORE_READ() macro generates slightly suboptimal code. If > there are intermediate pointers to be read, initial source pointer is > going to be assigned into a temporary variable and then temporary > variable is going to be uniformly used as a "source" pointer for all > intermediate pointer reads. Schematically (ignoring all the type casts), > BPF_CORE_READ(s, a, b, c) is expanded into: > ({ > const void *__t = src; > bpf_probe_read(&__t, sizeof(*__t), &__t->a); > bpf_probe_read(&__t, sizeof(*__t), &__t->b); > > typeof(s->a->b->c) __r; > bpf_probe_read(&__r, sizeof(*__r), &__t->c); > }) > > This initial `__t = src` makes calls more uniform, but causes slightly > less optimal register usage sometimes when compiled with Clang. This can > cascase into, e.g., more register spills. > > This patch fixes this issue by generating more optimal sequence: > ({ > const void *__t; > bpf_probe_read(&__t, sizeof(*__t), &src->a); /* <-- src here */ > bpf_probe_read(&__t, sizeof(*__t), &__t->b); > > typeof(s->a->b->c) __r; > bpf_probe_read(&__r, sizeof(*__r), &__t->c); > }) > > Fixes: 7db3822ab991 ("libbpf: Add BPF_CORE_READ/BPF_CORE_READ_INTO helpers") > Signed-off-by: Andrii Nakryiko Applied, thanks!