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SubjectRe: [PATCH v7 18/18] x86/fsgsbase/64: Add documentation for FSGSBASE
On Wed, 8 May 2019, Chang S. Bae wrote:

> Subject: x86/fsgsbase/64: Add documentation for FSGSBASE

The proper prefix is Documentation/x86:

> 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

Please move the vX annoations below the --- marker so they are stripped out
automatically and I don't have to do it manually. They are not part of the
final changelog.

> Documentation/x86/fsgs.txt | 103 +++++++++++++++++++++++++++++++++++++++++++++

The x86 documentation got converted to RST recently. Also as this is a
64bit specific documentation it belongs into Documentation/x86/x86_64 and
not into the generic x86 directory.

> +++ b/Documentation/x86/fsgs.txt
> @@ -0,0 +1,103 @@
> +

Documentation files require a SPDX license identifier as any other file.

> +Using FS and GS prefixes on 64-bit x86 linux

Moving this into the 64 bit specific folder spares all the 'oh this is
64bit only' notices all over the place.

> +
> +The x86 architecture supports segment prefixes per instruction to add an

per instruction? It's only for instructions which access memory, not for
instructions which are purely register based.

> +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.

That sentence does not make any sense. What has this to do with global

> +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

TLS is Thread Local Storage not Segment.

> +using thread. The compiler then automatically generates the correct prefixes

What means: declared using thread? I assume you meant declared with the
__thread storage class specifier. If so, why not using the proper technical

> +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

I don't see a tradition here and 'could' is just wrong.

> +
> + 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.]

So why is it even mentioned when it's not longer existing?

> +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.

User space threading libraries are one particular use case and not really
interesting for documenting this functionality. Documentation is about the
concepts and not about what a particular usecase prefers.

> 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.

I don't see the value of this either.

> +Newer Intel CPUs (Ivy Bridge and later) added new instructions to directly
> +access these registers quickly from user context:

The CPUs added new instructions?

> + 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 <immintrin.h> 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.


This is completely unstructured information hastily cobbled together.

As time is pressing for the 5.3 merge window, I reworked the documentation
as below. Please review and comment ASAP so I can merge the whole lot.



From: Thomas Gleixner <>
Subject: Documentation/x86/64: Add documentation for GS/FS addressing mode
Date: Thu, 13 Jun 2019 22:04:24 +0300

Originally-by: Andi Kleen <>
Signed-off-by: Thomas Gleixner <>
Documentation/x86/x86_64/fsgs.rst | 200 +++++++++++++++++++++++++++++++++++++
Documentation/x86/x86_64/index.rst | 1
2 files changed, 201 insertions(+)
create mode 100644 Documentation/x86/fsgs.txt

--- /dev/null
+++ b/Documentation/x86/x86_64/fsgs.rst
@@ -0,0 +1,200 @@
+.. 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. There is no storage class specifier similar to __thread which
+would cause the compiler to use GS based addressing modes. Newer versions
+of 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 is not enabling them
+ automatically. The kernel has to enable them explicitely 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 */
+ #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 6 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;
+ print("data0 = %ld\n", *ptr);
+ /* Set GS to point to data1 */
+ _writegsbase_u64(&data1);
+ /* ptr still addresses offset 0! */
+ print("data1 = %ld\n", *ptr);
+Clang does not provide these address space identifiers, but it provides
+an attribute based mechanism:
+ ==================================== =====================================
+ __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
--- a/Documentation/x86/x86_64/index.rst
+++ b/Documentation/x86/x86_64/index.rst
@@ -14,3 +14,4 @@ x86_64 Support
+ fsgs

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