Re: [RESEND PATCH v6 12/12] x86/fsgsbase/64: Add documentation for FSGSBASE

[Randy Dunlap]

On 3/15/19 1:06 PM, Chang S. Bae wrote:
> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> 
> v2: Minor updates to documentation requested in review.
> v3: Update for new gcc and various improvements.
> 
> Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> ---
>  Documentation/x86/fsgs.txt | 104 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 104 insertions(+)
>  create mode 100644 Documentation/x86/fsgs.txt
> 

Globally s/64bit/64-bit/ and s/32bit/32-bit/.

More comments below.

> 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

                                                               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

   However,

> +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

                                            context.  {or 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.

                  intrinsics,

> +
> +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)

add ending '.' above.

> +
> +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

                                        does not

> +they will fault with an #UD exception.

well, I would say "with a #UD exception."

> +
> +An FSGSBASE enabled kernel can be detected by checking the AT_HWCAP2

      FSGSBASE-enabled

> +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

            CPUID check is needed

> +does not support it.
> +
> +gcc 6 will have special support to directly access data relative

"will have"? future? or:

   gcc 6 has special support

> +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
> 


-- 
~Randy