Messages in this thread | | | Date | Mon, 20 Oct 2014 16:28:53 -0400 | From | Konrad Rzeszutek Wilk <> | Subject | Re: [PATCH 7/8] x86: Add documentation for rd/wr fs/gs base |
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On Tue, Oct 14, 2014 at 10:11:50PM -0700, 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..d895b25 > --- /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 > +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
s/the/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 > + > + 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.
Could you explain that a bit more? > + > +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.
#GP? or #UD?
> + > +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 > -- > 1.9.3 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/
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