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From: tip-bot for Thomas Gleixner <tipbot@zytor.com>
To: linux-tip-commits@vger.kernel.org
Cc: mingo@kernel.org, hpa@zytor.com, corbet@lwn.net,
	rdunlap@infradead.org, chang.seok.bae@intel.com,
	linux-kernel@vger.kernel.org, ak@linux.intel.com,
	tglx@linutronix.de, ravi.v.shankar@intel.com
Subject: [tip:x86/cpu] Documentation/x86/64: Add documentation for GS/FS addressing mode
Date: Sat, 22 Jun 2019 03:15:32 -0700	[thread overview]
Message-ID: <tip-2c7b5ac5d5a93c4b0557293d06c6677f765081a6@git.kernel.org> (raw)
In-Reply-To: <alpine.DEB.2.21.1906132246310.1791@nanos.tec.linutronix.de>

Commit-ID:  2c7b5ac5d5a93c4b0557293d06c6677f765081a6
Gitweb:     https://git.kernel.org/tip/2c7b5ac5d5a93c4b0557293d06c6677f765081a6
Author:     Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Thu, 13 Jun 2019 22:04:24 +0300
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Sat, 22 Jun 2019 11:38:57 +0200

Documentation/x86/64: Add documentation for GS/FS addressing mode

Explain how the GS/FS based addressing can be utilized in user space
applications along with the differences between the generic prctl() based
GS/FS base control and the FSGSBASE version available on newer CPUs.

Originally-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "Bae, Chang Seok" <chang.seok.bae@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>,
Cc: H . Peter Anvin <hpa@zytor.com>
Cc: "Shankar, Ravi V" <ravi.v.shankar@intel.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: https://lkml.kernel.org/r/alpine.DEB.2.21.1906132246310.1791@nanos.tec.linutronix.de
---
 Documentation/x86/x86_64/fsgs.rst  | 199 +++++++++++++++++++++++++++++++++++++
 Documentation/x86/x86_64/index.rst |   1 +
 2 files changed, 200 insertions(+)

diff --git a/Documentation/x86/x86_64/fsgs.rst b/Documentation/x86/x86_64/fsgs.rst
new file mode 100644
index 000000000000..380c0b5ccca2
--- /dev/null
+++ b/Documentation/x86/x86_64/fsgs.rst
@@ -0,0 +1,199 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Using FS and GS segments in user space applications
+===================================================
+
+The x86 architecture supports segmentation. Instructions which access
+memory can use segment register based addressing mode. The following
+notation is used to address a byte within a segment:
+
+  Segment-register:Byte-address
+
+The segment base address is added to the Byte-address to compute the
+resulting virtual address which is accessed. This allows to access multiple
+instances of data with the identical Byte-address, i.e. the same code. The
+selection of a particular instance is purely based on the base-address in
+the segment register.
+
+In 32-bit mode the CPU provides 6 segments, which also support segment
+limits. The limits can be used to enforce address space protections.
+
+In 64-bit mode the CS/SS/DS/ES segments are ignored and the base address is
+always 0 to provide a full 64bit address space. The FS and GS segments are
+still functional in 64-bit mode.
+
+Common FS and GS usage
+------------------------------
+
+The FS segment is commonly used to address Thread Local Storage (TLS). FS
+is usually managed by runtime code or a threading library. Variables
+declared with the '__thread' storage class specifier are instantiated per
+thread and the compiler emits the FS: address prefix for accesses to these
+variables. Each thread has its own FS base address so common code can be
+used without complex address offset calculations to access the per thread
+instances. Applications should not use FS for other purposes when they use
+runtimes or threading libraries which manage the per thread FS.
+
+The GS segment has no common use and can be used freely by
+applications. GCC and Clang support GS based addressing via address space
+identifiers.
+
+Reading and writing the FS/GS base address
+------------------------------------------
+
+There exist two mechanisms to read and write the FS/FS base address:
+
+ - the arch_prctl() system call
+
+ - the FSGSBASE instruction family
+
+Accessing FS/GS base with arch_prctl()
+--------------------------------------
+
+ The arch_prctl(2) based mechanism is available on all 64bit CPUs and all
+ kernel versions.
+
+ Reading the base:
+
+   arch_prctl(ARCH_GET_FS, &fsbase);
+   arch_prctl(ARCH_GET_GS, &gsbase);
+
+ Writing the base:
+
+   arch_prctl(ARCH_SET_FS, fsbase);
+   arch_prctl(ARCH_SET_GS, gsbase);
+
+ The ARCH_SET_GS prctl may be disabled depending on kernel configuration
+ and security settings.
+
+Accessing FS/GS base with the FSGSBASE instructions
+---------------------------------------------------
+
+ With the Ivy Bridge CPU generation Intel introduced a new set of
+ instructions to access the FS and GS base registers directly from user
+ space. These instructions are also supported on AMD Family 17H CPUs. The
+ following instructions are available:
+
+  =============== ===========================
+  RDFSBASE %reg   Read the FS base register
+  RDGSBASE %reg   Read the GS base register
+  WRFSBASE %reg   Write the FS base register
+  WRGSBASE %reg   Write the GS base register
+  =============== ===========================
+
+ The instructions avoid the overhead of the arch_prctl() syscall and allow
+ more flexible usage of the FS/GS addressing modes in user space
+ applications. This does not prevent conflicts between threading libraries
+ and runtimes which utilize FS and applications which want to use it for
+ their own purpose.
+
+FSGSBASE instructions enablement
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+ The instructions are enumerated in CPUID leaf 7, bit 0 of EBX. If
+ available /proc/cpuinfo shows 'fsgsbase' in the flag entry of the CPUs.
+
+ The availability of the instructions does not enable them
+ automatically. The kernel has to enable them explicitly in CR4. The
+ reason for this is that older kernels make assumptions about the values in
+ the GS register and enforce them when GS base is set via
+ arch_prctl(). Allowing user space to write arbitrary values to GS base
+ would violate these assumptions and cause malfunction.
+
+ On kernels which do not enable FSGSBASE the execution of the FSGSBASE
+ instructions will fault with a #UD exception.
+
+ The kernel provides reliable information about the enabled state in the
+ ELF AUX vector. If the HWCAP2_FSGSBASE bit is set in the AUX vector, the
+ kernel has FSGSBASE instructions enabled and applications can use them.
+ The following code example shows how this detection works::
+
+   #include <sys/auxv.h>
+   #include <elf.h>
+
+   /* Will be eventually in asm/hwcap.h */
+   #ifndef HWCAP2_FSGSBASE
+   #define HWCAP2_FSGSBASE        (1 << 1)
+   #endif
+
+   ....
+
+   unsigned val = getauxval(AT_HWCAP2);
+
+   if (val & HWCAP2_FSGSBASE)
+        printf("FSGSBASE enabled\n");
+
+FSGSBASE instructions compiler support
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+GCC version 4.6.4 and newer provide instrinsics for the FSGSBASE
+instructions. Clang supports them as well.
+
+  =================== ===========================
+  _readfsbase_u64()   Read the FS base register
+  _readfsbase_u64()   Read the GS base register
+  _writefsbase_u64()  Write the FS base register
+  _writegsbase_u64()  Write the GS base register
+  =================== ===========================
+
+To utilize these instrinsics <immintrin.h> must be included in the source
+code and the compiler option -mfsgsbase has to be added.
+
+Compiler support for FS/GS based addressing
+-------------------------------------------
+
+GCC version 6 and newer provide support for FS/GS based addressing via
+Named Address Spaces. GCC implements the following address space
+identifiers for x86:
+
+  ========= ====================================
+  __seg_fs  Variable is addressed relative to FS
+  __seg_gs  Variable is addressed relative to GS
+  ========= ====================================
+
+The preprocessor symbols __SEG_FS and __SEG_GS are defined when these
+address spaces are supported. Code which implements fallback modes should
+check whether these symbols are defined. Usage example::
+
+  #ifdef __SEG_GS
+
+  long data0 = 0;
+  long data1 = 1;
+
+  long __seg_gs *ptr;
+
+  /* Check whether FSGSBASE is enabled by the kernel (HWCAP2_FSGSBASE) */
+  ....
+
+  /* Set GS to point to data0 */
+  _writegsbase_u64(&data0);
+
+  /* Access offset 0 of GS */
+  ptr = 0;
+  printf("data0 = %ld\n", *ptr);
+
+  /* Set GS to point to data1 */
+  _writegsbase_u64(&data1);
+  /* ptr still addresses offset 0! */
+  printf("data1 = %ld\n", *ptr);
+
+
+Clang does not provide the GCC address space identifiers, but it provides
+address spaces via an attribute based mechanism in Clang 5 and newer
+versions:
+
+ ==================================== =====================================
+  __attribute__((address_space(256))  Variable is addressed relative to GS
+  __attribute__((address_space(257))  Variable is addressed relative to FS
+ ==================================== =====================================
+
+FS/GS based addressing with inline assembly
+-------------------------------------------
+
+In case the compiler does not support address spaces, inline assembly can
+be used for FS/GS based addressing mode::
+
+	mov %fs:offset, %reg
+	mov %gs:offset, %reg
+
+	mov %reg, %fs:offset
+	mov %reg, %gs:offset
diff --git a/Documentation/x86/x86_64/index.rst b/Documentation/x86/x86_64/index.rst
index d6eaaa5a35fc..a56070fc8e77 100644
--- a/Documentation/x86/x86_64/index.rst
+++ b/Documentation/x86/x86_64/index.rst
@@ -14,3 +14,4 @@ x86_64 Support
    fake-numa-for-cpusets
    cpu-hotplug-spec
    machinecheck
+   fsgs

      parent reply	other threads:[~2019-06-22 10:15 UTC|newest]

Thread overview: 63+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-08 10:02 [PATCH v7 00/18] x86: Enable FSGSBASE instructions Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 01/18] x86/fsgsbase/64: Fix ARCH_SET_FS/GS behaviors for a remote task Chang S. Bae
     [not found]   ` <74F4F506-2913-4013-9D81-A0C69FA8CDF1@intel.com>
     [not found]     ` <6420E1A5-B5AD-4028-AA91-AA4D5445AC83@intel.com>
2019-06-16 15:44       ` Bae, Chang Seok
2019-06-16 16:32         ` Thomas Gleixner
2019-06-22 10:04         ` [tip:x86/cpu] x86/ptrace: Prevent ptrace from clearing the FS/GS selector tip-bot for Chang S. Bae
2020-06-18 13:50         ` [tip: x86/fsgsbase] " tip-bot2 for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 02/18] selftests/x86/fsgsbase: Test ptracer-induced GSBASE write Chang S. Bae
2019-06-22 10:04   ` [tip:x86/cpu] " tip-bot for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 03/18] x86/fsgsbase/64: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE Chang S. Bae
2019-06-22 10:05   ` [tip:x86/cpu] x86/cpu: " tip-bot for Andy Lutomirski
2020-06-18 13:50   ` [tip: x86/fsgsbase] " tip-bot2 for Andy Lutomirski
2019-05-08 10:02 ` [PATCH v7 04/18] kbuild: Raise the minimum required binutils version to 2.21 Chang S. Bae
2019-06-22 10:06   ` [tip:x86/cpu] " tip-bot for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 05/18] x86/fsgsbase/64: Add intrinsics for FSGSBASE instructions Chang S. Bae
2019-06-22 10:06   ` [tip:x86/cpu] " tip-bot for Andi Kleen
2020-06-18 13:50   ` [tip: x86/fsgsbase] " tip-bot2 for Andi Kleen
2019-05-08 10:02 ` [PATCH v7 06/18] x86/fsgsbase/64: Enable FSGSBASE instructions in the helper functions Chang S. Bae
2019-06-22 10:07   ` [tip:x86/cpu] x86/fsgsbase/64: Enable FSGSBASE instructions in " tip-bot for Chang S. Bae
2020-06-18 13:50   ` [tip: x86/fsgsbase] " tip-bot2 for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 07/18] x86/fsgsbase/64: Preserve FS/GS state in __switch_to() if FSGSBASE is on Chang S. Bae
2019-06-21 15:22   ` Thomas Gleixner
2019-06-22 10:08   ` [tip:x86/cpu] x86/process/64: Use FSBSBASE in switch_to() if available tip-bot for Andy Lutomirski
2020-06-18 13:50   ` [tip: x86/fsgsbase] " tip-bot2 for Andy Lutomirski
2019-05-08 10:02 ` [PATCH v7 08/18] x86/fsgsbase/64: When copying a thread, use the FSGSBASE instructions Chang S. Bae
2019-06-22 10:09   ` [tip:x86/cpu] x86/process/64: Use FSGSBASE instructions on thread copy and ptrace tip-bot for Chang S. Bae
2020-06-18 13:50   ` [tip: x86/fsgsbase] " tip-bot2 for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 09/18] x86/entry/64: Add the READ_MSR_GSBASE macro Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 10/18] x86/entry/64: Switch CR3 before SWAPGS on the paranoid entry Chang S. Bae
2019-06-22 10:09   ` [tip:x86/cpu] x86/entry/64: Switch CR3 before SWAPGS in " tip-bot for Chang S. Bae
2020-06-18 13:50   ` [tip: x86/fsgsbase] " tip-bot2 for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 11/18] x86/fsgsbase/64: Introduce the FIND_PERCPU_BASE macro Chang S. Bae
2019-06-22 10:10   ` [tip:x86/cpu] x86/entry/64: " tip-bot for Chang S. Bae
2020-06-18 13:50   ` [tip: x86/fsgsbase] " tip-bot2 for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 12/18] x86/fsgsbase/64: GSBASE handling with FSGSBASE in the paranoid path Chang S. Bae
2019-06-22 10:11   ` [tip:x86/cpu] x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit tip-bot for Chang S. Bae
2019-06-29  7:21   ` [PATCH v7 12/18] x86/fsgsbase/64: GSBASE handling with FSGSBASE in the paranoid path Bae, Chang Seok
2019-06-29  7:37     ` Thomas Gleixner
2020-06-18 13:50   ` [tip: x86/fsgsbase] x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit tip-bot2 for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 13/18] x86/fsgsbase/64: Document GSBASE handling in the paranoid path Chang S. Bae
2019-06-22 10:11   ` [tip:x86/cpu] x86/entry/64: " tip-bot for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 14/18] selftests/x86/fsgsbase: Test WRGSBASE Chang S. Bae
2019-06-22 10:12   ` [tip:x86/cpu] selftests/x86/fsgsbase: Test RD/WRGSBASE tip-bot for Andy Lutomirski
2019-05-08 10:02 ` [PATCH v7 15/18] selftests/x86/fsgsbase: Test ptracer-induced GSBASE write with FSGSBASE Chang S. Bae
2019-06-22 10:13   ` [tip:x86/cpu] " tip-bot for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 16/18] x86/fsgsbase/64: Enable FSGSBASE by default and add a chicken bit Chang S. Bae
2019-06-22 10:14   ` [tip:x86/cpu] x86/cpu: Enable FSGSBASE on 64bit " tip-bot for Andy Lutomirski
2020-06-18 13:50   ` [tip: x86/fsgsbase] " tip-bot2 for Andy Lutomirski
2019-05-08 10:02 ` [PATCH v7 17/18] x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2 Chang S. Bae
2019-06-22 10:14   ` [tip:x86/cpu] " tip-bot for Andi Kleen
2020-06-18 13:50   ` [tip: x86/fsgsbase] " tip-bot2 for Andi Kleen
2019-05-08 10:02 ` [PATCH v7 18/18] x86/fsgsbase/64: Add documentation for FSGSBASE Chang S. Bae
2019-06-14  6:54   ` Thomas Gleixner
2019-06-14 20:07     ` Bae, Chang Seok
     [not found]       ` <89BE934A-A392-4CED-83E5-CA4FADDAE6DF@intel.com>
2019-06-16  8:39         ` Thomas Gleixner
2019-06-16 12:34           ` Thomas Gleixner
2019-06-16 15:34             ` Bae, Chang Seok
2019-06-16 16:05               ` Thomas Gleixner
2019-06-16 20:48                 ` Bae, Chang Seok
2019-06-16 22:00                   ` Thomas Gleixner
     [not found]                     ` <8E2E84B6-BCCC-424D-A1A7-604828B389FB@intel.com>
2019-06-17  5:18                       ` Thomas Gleixner
2019-06-16 15:54     ` Randy Dunlap
2019-06-16 16:07       ` Thomas Gleixner
2019-06-22 10:15     ` tip-bot for Thomas Gleixner [this message]

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