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 25EE4C04A6B for ; Wed, 8 May 2019 17:03:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 007E421530 for ; Wed, 8 May 2019 17:03:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729049AbfEHRDX (ORCPT ); Wed, 8 May 2019 13:03:23 -0400 Received: from mga03.intel.com ([134.134.136.65]:5321 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728927AbfEHRDG (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="169697610" 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 , Randy Dunlap Subject: [PATCH v7 18/18] x86/fsgsbase/64: Add documentation for FSGSBASE Date: Wed, 8 May 2019 03:02:33 -0700 Message-Id: <1557309753-24073-19-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 v2: Minor updates to documentation requested in review. v3: Update for new gcc and various improvements. v4: Address the typos pointed by Randy Dunlap Signed-off-by: Andi Kleen Signed-off-by: Chang S. Bae Cc: Randy Dunlap Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Andy Lutomirski Cc: H. Peter Anvin --- Documentation/x86/fsgs.txt | 103 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 103 insertions(+) create mode 100644 Documentation/x86/fsgs.txt diff --git a/Documentation/x86/fsgs.txt b/Documentation/x86/fsgs.txt new file mode 100644 index 0000000..a6e2e38 --- /dev/null +++ b/Documentation/x86/fsgs.txt @@ -0,0 +1,103 @@ + +Using FS and GS prefixes on 64-bit x86 linux + +The x86 architecture supports segment prefixes per instruction to add an +offset to an address. On 64-bit x86, these are mostly nops, except for FS +and GS. + +This offers an efficient way to reference a global pointer. + +The compiler has to generate special code to use these base registers, +or they can be accessed with inline assembler. + + mov %gs:offset,%reg + mov %fs:offset,%reg + +On 64-bit code, FS is used to address the thread local segment (TLS), declared +using thread. The compiler then automatically generates the correct prefixes +and relocations to access these values. + +FS is normally managed by the runtime code or the threading library. +Overwriting it can break a lot of things (including syscalls and gdb), +but it can make sense to save/restore it for threading purposes. + +GS is freely available, but may need special (compiler or inline assembler) +code to use. + +Traditionally 64-bit FS and GS could be set by the arch_prctl system call + + arch_prctl(ARCH_SET_GS, value) + arch_prctl(ARCH_SET_FS, value) + +[There was also an older method using modify_ldt(), inherited from 32-bit, +but this is not discussed here.] + +However, using a syscall is problematic for user space threading libraries +that want to context switch in user space. The whole point of them +is avoiding the overhead of a syscall. It's also cleaner for compilers +wanting to use the extra register to use instructions to write +it, or read it directly to compute addresses and offsets. + +Newer Intel CPUs (Ivy Bridge and later) added new instructions to directly +access these registers quickly from user context: + + RDFSBASE %reg read the FS base (or _readfsbase_u64) + RDGSBASE %reg read the GS base (or _readgsbase_u64) + + WRFSBASE %reg write the FS base (or _writefsbase_u64) + WRGSBASE %reg write the GS base (or _writegsbase_u64) + +If you use the intrinsics, include and set the -mfsgsbase option. + +The instructions are supported by the CPU when the "fsgsbase" string is shown +in /proc/cpuinfo (or directly retrieved through the CPUID instruction, +7:0 (ebx), word 9, bit 0). + +The instructions are only available to 64-bit binaries. + +In addition the kernel needs to explicitly enable these instructions, as it +may otherwise not correctly context switch the state. Newer Linux +kernels enable this. When the kernel does not enable the instruction +they will fault with a #UD exception. + +An FSGSBASE-enabled kernel can be detected by checking the AT_HWCAP2 +bitmask in the aux vector. When the HWCAP2_FSGSBASE bit is set the +kernel supports FSGSBASE. + + #include + #include + + /* Will be eventually in asm/hwcap.h */ + #define HWCAP2_FSGSBASE (1 << 1) + + unsigned val = getauxval(AT_HWCAP2); + if (val & HWCAP2_FSGSBASE) { + asm("wrgsbase %0" :: "r" (ptr)); + } + +No extra CPUID check is needed as the kernel will not set this bit if the CPU +does not support it. + +gcc 6 has special support to directly access data relative to fs/gs using the +__seg_fs and __seg_gs address space pointer modifiers. + +#ifndef __SEG_GS +#error "Need gcc 6 or later" +#endif + +struct gsdata { + int a; + int b; +} gsdata = { 1, 2 }; + +int __seg_gs *valp = 0; /* offset relative to GS */ + + /* Check if kernel supports FSGSBASE as above */ + + /* Set up new GS */ + asm("wrgsbase %0" :: "r" (&gsdata)); + + /* Now the global pointer can be used normally */ + printf("gsdata.a = %d\n", *valp); + +Andi Kleen -- 2.7.4