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=-7.8 required=3.0 tests=DATE_IN_PAST_06_12, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_PASS,URIBL_BLOCKED,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 76871C04AAB for ; Wed, 8 May 2019 17:03:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 52AAD21530 for ; Wed, 8 May 2019 17:03:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729059AbfEHRD3 (ORCPT ); Wed, 8 May 2019 13:03:29 -0400 Received: from mga03.intel.com ([134.134.136.65]:5322 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728996AbfEHRDG (ORCPT ); Wed, 8 May 2019 13:03:06 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga002.fm.intel.com ([10.253.24.26]) by orsmga103.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 08 May 2019 10:03:05 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.60,446,1549958400"; d="scan'208";a="169697606" Received: from chang-linux-3.sc.intel.com ([172.25.66.171]) by fmsmga002.fm.intel.com with ESMTP; 08 May 2019 10:03:05 -0700 From: "Chang S. Bae" To: Andy Lutomirski , Thomas Gleixner , Ingo Molnar , "H . Peter Anvin" , Andi Kleen Cc: Ravi Shankar , "Chang S . Bae" , LKML Subject: [PATCH v7 17/18] x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2 Date: Wed, 8 May 2019 03:02:32 -0700 Message-Id: <1557309753-24073-18-git-send-email-chang.seok.bae@intel.com> X-Mailer: git-send-email 2.7.4 In-Reply-To: <1557309753-24073-1-git-send-email-chang.seok.bae@intel.com> References: <1557309753-24073-1-git-send-email-chang.seok.bae@intel.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Andi Kleen The kernel needs to explicitly enable FSGSBASE. So, the application needs to know if it can safely use these instructions. Just looking at the CPUID bit is not enough because it may be running in a kernel that does not enable the instructions. One way for the application would be to just try and catch the SIGILL. But that is difficult to do in libraries which may not want to overwrite the signal handlers of the main application. So we need to provide a way for the application to discover the kernel capability. I used AT_HWCAP2 in the ELF aux vector which is already used by PPC for similar things. We define a new Linux defined bitmap returned in AT_HWCAP. Next to MONITOR/MWAIT, bit 1 is reserved for FSGSBASE capability checks. The application can then access it manually or using the getauxval() function in newer glibc. Signed-off-by: Andi Kleen Signed-off-by: Chang S. Bae Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Andy Lutomirski Cc: H. Peter Anvin --- arch/x86/include/uapi/asm/hwcap2.h | 3 +++ arch/x86/kernel/cpu/common.c | 4 +++- 2 files changed, 6 insertions(+), 1 deletion(-) diff --git a/arch/x86/include/uapi/asm/hwcap2.h b/arch/x86/include/uapi/asm/hwcap2.h index 6ebaae9..c5ce54e 100644 --- a/arch/x86/include/uapi/asm/hwcap2.h +++ b/arch/x86/include/uapi/asm/hwcap2.h @@ -5,4 +5,7 @@ /* MONITOR/MWAIT enabled in Ring 3 */ #define HWCAP2_RING3MWAIT (1 << 0) +/* Kernel allows FSGSBASE instructions available in Ring 3 */ +#define HWCAP2_FSGSBASE BIT(1) + #endif diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c index fd7ef49..f8b1d7a 100644 --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -1361,8 +1361,10 @@ static void identify_cpu(struct cpuinfo_x86 *c) setup_umip(c); /* Enable FSGSBASE instructions if available. */ - if (cpu_has(c, X86_FEATURE_FSGSBASE)) + if (cpu_has(c, X86_FEATURE_FSGSBASE)) { cr4_set_bits(X86_CR4_FSGSBASE); + elf_hwcap2 |= HWCAP2_FSGSBASE; + } /* * The vendor-specific functions might have changed features. -- 2.7.4