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=-18.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=ham 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 C295DC433EF for ; Wed, 8 Sep 2021 17:17:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AB0B861157 for ; Wed, 8 Sep 2021 17:17:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1352443AbhIHRS6 (ORCPT ); Wed, 8 Sep 2021 13:18:58 -0400 Received: from terminus.zytor.com ([198.137.202.136]:59317 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230217AbhIHRS5 (ORCPT ); Wed, 8 Sep 2021 13:18:57 -0400 Received: from tazenda.hos.anvin.org ([IPv6:2601:646:8600:3c70:7285:c2ff:fefb:fd4]) (authenticated bits=0) by mail.zytor.com (8.16.1/8.15.2) with ESMTPSA id 188HHQGV4133488 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NO); Wed, 8 Sep 2021 10:17:37 -0700 DKIM-Filter: OpenDKIM Filter v2.11.0 mail.zytor.com 188HHQGV4133488 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zytor.com; s=2021083001; t=1631121457; bh=+c97iHdGu2Ow8GnHPTGKXxAd8Rj1up2lNHoffmQQRHg=; h=From:To:Cc:Subject:Date:From; b=Kh9NjNB5WYTcSaV/cpFT7ZNRJfYHo2m1z2AoLCJbHfiLWZAAfAhwJKeghyIbXSpwC JvPjrrUkNc7q6BUd3YNLgCSU8dmQWuVnTpL4sRDf+2FWbf1ENHdFRCQWRwc16d5Jz2 Fn+YcDF1MnaJKL7qbzdTbhJTyOzY26zL2cxDK4U+r/xFwzC0OAdHAF2Tour8NKSR1k U04KjQ8Nunc6gep2LLU5N/nDOB2AwziuwgvbqmtoD5G1Mupn/yOwpKu+C67PM/1sAi qKqj7Y2a8sbXVyXKZMt/JoqCoBLe/XpELqOC5g5CWjlKyRgz5QYjelrgLaWF21yS78 kPlsWN1jplZoQ== From: "H. Peter Anvin (Intel)" To: Thomas Gleixner , Ingo Molnar , Andy Lutomirski , Borislav Petkov Cc: Linux Kernel Mailing List , "H. Peter Anvin (Intel)" Subject: [PATCH] x86/asm: pessimize the pre-initialization case in static_cpu_has() Date: Wed, 8 Sep 2021 10:17:16 -0700 Message-Id: <20210908171716.3340120-1-hpa@zytor.com> X-Mailer: git-send-email 2.31.1 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org gcc will sometimes manifest the address of boot_cpu_data in a register as part of constant propagation. When multiple static_cpu_has() are used this may foul the mainline code with a register load which will only be used on the fallback path, which is unused after initialization. Explicitly force gcc to use immediate (rip-relative) addressing for the fallback path, thus removing any possible register use from static_cpu_has(). Signed-off-by: H. Peter Anvin (Intel) --- arch/x86/include/asm/cpufeature.h | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h index 16a51e7288d5..ff18906b60d8 100644 --- a/arch/x86/include/asm/cpufeature.h +++ b/arch/x86/include/asm/cpufeature.h @@ -173,20 +173,25 @@ extern void clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int bit); * means that the boot_cpu_has() variant is already fast enough for the * majority of cases and you should stick to using it as it is generally * only two instructions: a RIP-relative MOV and a TEST. + * + * Do not use an "m" constraint for [cap_byte] here: gcc doesn't know + * that this is only used on a fallback path and will sometimes cause + * it to manifest the address of boot_cpu_data in a register, fouling + * the mainline (post-initialization) code. */ static __always_inline bool _static_cpu_has(u16 bit) { asm_volatile_goto( ALTERNATIVE_TERNARY("jmp 6f", %P[feature], "", "jmp %l[t_no]") - ".section .altinstr_aux,\"ax\"\n" + ".pushsection .altinstr_aux,\"ax\"\n" "6:\n" - " testb %[bitnum],%[cap_byte]\n" + " testb %[bitnum],%P[cap_byte]\n" " jnz %l[t_yes]\n" " jmp %l[t_no]\n" - ".previous\n" + ".popsection\n" : : [feature] "i" (bit), [bitnum] "i" (1 << (bit & 7)), - [cap_byte] "m" (((const char *)boot_cpu_data.x86_capability)[bit >> 3]) + [cap_byte] "i" (&((const char *)boot_cpu_data.x86_capability)[bit >> 3]) : : t_yes, t_no); t_yes: return true; -- 2.31.1