Re: [PATCH 7/7] x86: Add documentation for rd/wr fs/gs base

Randy Dunlap
2014-04-29T02:23:58Z

On 04/28/14 15:12, Andi Kleen wrote:
> From: Andi Kleen <ak@linux.intel.com>
>
> Signed-off-by: Andi Kleen <ak@linux.intel.com>
> ---
> Documentation/x86/fsgs.txt | 73 ++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 73 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..0a1755f
> --- /dev/null
> +++ b/Documentation/x86/fsgs.txt
> @@ -0,0 +1,73 @@
> +
> +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

64-bit

> +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 the automatically generates the correct prefixes and

then

> +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

64-bit

> +
> + arch_prctl(ARCH_SET_GS, value)
> + arch_prctl(ARCH_SET_FS, value)
> +
> +[There was also an older method using modify_ldt(), inherited from 32bit,

32-bit,

> +but this is not discussed here.]
> +
> +However using a syscall is problematic for user space threading libraries

However,

> +that want to context switch in user space.
> +
> +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.

64-bit

> +
> +However the kernel needs to explicitely enable these instructions, as it

However, explicitly

> +may otherwise not correctly context switch the state. Newer Linux
> +kernels enable this. When the kernel did not enable the instruction
> +they will fault.
> +
> +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 explicitely)

explicitly).

> +
> +
> +Andi Kleen
>

--
~Randy