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Subject[PATCH v7 18/18] x86/fsgsbase/64: Add documentation for FSGSBASE
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 <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.
+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 <sys/auxv.h>
+ #include <elf.h>
+ /* 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"
+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
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