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Subject[PATCH 7/8] x86: Add documentation for rd/wr fs/gs base
From: Andi Kleen <>

v2: Minor updates to documentation requested in review.
Signed-off-by: Andi Kleen <>
Documentation/x86/fsgs.txt | 76 ++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 76 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..26a1e29
--- /dev/null
+++ b/Documentation/x86/fsgs.txt
@@ -0,0 +1,76 @@
+Using FS and GS prefixes on x86_64-linux
+The x86 architecture supports segment prefixes per instruction to add an
+offset to an address. On 64bit 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
+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.
+GS is freely available, but may need special (compiler or inline assembler)
+code to use.
+Traditionally 64bit 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 32bit,
+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)
+The instructions are supported by the CPU when the "fsgsbase" string is shown in
+/proc/cpuinfo (or directly retrieved through the CPUID instruction).
+The instructions are only available to 64bit binaries.
+However 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 did not enable the instruction
+they will fault with an #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 RDFSGSBASE.
+ #include <sys/auxv.h>
+ #include <elf.h>
+ /* Will be eventually in asm/hwcap.h */
+ #define HWCAP2_FSGSBASE (1 << 0)
+ unsigned val = getauxval(AT_HWCAP2);
+ if (val & HWCAP2_FSGSBASE) {
+ asm("wrgsbase %0" :: "r" (ptr));
+ }
+Another requirement is that the FS or GS selector has to be zero
+(is normally true unless changed explicitly)
+Andi Kleen

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