From: Andi Kleen <a...@linux.intel.com> v2: Minor updates to documentation requested in review. v3: Update for new gcc and various improvements.
[ chang: Fix some typo. Fix the example code. ] Signed-off-by: Andi Kleen <a...@linux.intel.com> Signed-off-by: Chang S. Bae <chang.seok....@intel.com> Cc: Andy Lutomirski <l...@kernel.org> Cc: H. Peter Anvin <h...@zytor.com> Cc: Thomas Gleixner <t...@linutronix.de> Cc: Ingo Molnar <mi...@kernel.org> --- Documentation/x86/fsgs.txt | 104 +++++++++++++++++++++++++++++++++++++ 1 file changed, 104 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 000000000000..7a973a5c1767 --- /dev/null +++ b/Documentation/x86/fsgs.txt @@ -0,0 +1,104 @@ + +Using FS and GS prefixes on 64bit x86 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 + +On 64bit 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 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) + +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 64bit 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 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 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 needed as the kernel will not set this bit if the CPU +does not support it. + +gcc 6 will have 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 -- 2.19.1