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From: Randy Dunlap <rdunlap@infradead.org>
To: "Chang S. Bae" <chang.seok.bae@intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>, Andy Lutomirski <luto@kernel.org>,
	"H . Peter Anvin" <hpa@zytor.com>,
	Andi Kleen <ak@linux.intel.com>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RESEND PATCH v6 12/12] x86/fsgsbase/64: Add documentation for FSGSBASE
Date: Sat, 30 Mar 2019 09:15:57 -0700
Message-ID: <1bda8272-1f3b-7e9d-1065-e40cd9db6c4a@infradead.org> (raw)
In-Reply-To: <1552680405-5265-13-git-send-email-chang.seok.bae@intel.com>

On 3/15/19 1:06 PM, Chang S. Bae wrote:
> From: Andi Kleen <ak@linux.intel.com>
> 
> v2: Minor updates to documentation requested in review.
> v3: Update for new gcc and various improvements.
> 
> Signed-off-by: Andi Kleen <ak@linux.intel.com>
> Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> ---
>  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

  reply index

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-15 20:06 [RESEND PATCH v6 00/12] x86: Enable FSGSBASE instructions Chang S. Bae
2019-03-15 20:06 ` [RESEND PATCH v6 01/12] x86/fsgsbase/64: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE Chang S. Bae
2019-03-15 20:06 ` [RESEND PATCH v6 02/12] kbuild: Raise the minimum required binutils version to 2.21 Chang S. Bae
2019-03-15 20:06 ` [RESEND PATCH v6 03/12] x86/fsgsbase/64: Add intrinsics for FSGSBASE instructions Chang S. Bae
2019-03-15 20:06 ` [RESEND PATCH v6 04/12] x86/fsgsbase/64: Enable FSGSBASE instructions in the helper functions Chang S. Bae
2019-03-25 11:38   ` Thomas Gleixner
2019-03-25 12:46     ` Thomas Gleixner
2019-03-25 13:05       ` Thomas Gleixner
2019-03-26  0:38     ` Andi Kleen
2019-03-26 15:01       ` New feature/ABI review process [was Re: [RESEND PATCH v6 04/12] x86/fsgsbase/64:..] Thomas Gleixner
2019-03-26 22:56         ` Andi Kleen
2019-03-27 21:15           ` Thomas Gleixner
2019-03-15 20:06 ` [RESEND PATCH v6 05/12] x86/fsgsbase/64: Preserve FS/GS state in __switch_to() if FSGSBASE is on Chang S. Bae
2019-03-15 20:06 ` [RESEND PATCH v6 06/12] x86/fsgsbase/64: When copying a thread, use the FSGSBASE instructions if available Chang S. Bae
2019-03-15 20:06 ` [RESEND PATCH v6 07/12] x86/fsgsbase/64: Introduce the FIND_PERCPU_BASE macro Chang S. Bae
2019-03-25  9:02   ` Thomas Gleixner
2019-05-01 13:52     ` Bae, Chang Seok
2019-03-15 20:06 ` [RESEND PATCH v6 08/12] x86/fsgsbase/64: Use the per-CPU base as GSBASE at the paranoid_entry Chang S. Bae
2019-03-25  9:44   ` Thomas Gleixner
2019-04-05  8:35     ` Thomas Gleixner
2019-04-05 13:50       ` Andy Lutomirski
2019-05-01 13:52         ` Bae, Chang Seok
2019-05-01 17:40           ` Andy Lutomirski
2019-05-01 18:01             ` Bae, Chang Seok
     [not found]               ` <7029A32B-958E-4C1E-8B5F-D49BA68E4755@intel.com>
2019-05-01 20:25                 ` Andy Lutomirski
2019-05-01 21:04                   ` Bae, Chang Seok
2019-05-02  0:29                     ` Andy Lutomirski
2019-05-06 22:56     ` Bae, Chang Seok
2019-03-15 20:06 ` [RESEND PATCH v6 09/12] selftests/x86/fsgsbase: Test WRGSBASE Chang S. Bae
2019-03-15 20:06 ` [RESEND PATCH v6 10/12] x86/fsgsbase/64: Enable FSGSBASE by default and add a chicken bit Chang S. Bae
2019-03-15 20:06 ` [RESEND PATCH v6 11/12] x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2 Chang S. Bae
2019-03-15 20:06 ` [RESEND PATCH v6 12/12] x86/fsgsbase/64: Add documentation for FSGSBASE Chang S. Bae
2019-03-30 16:15   ` Randy Dunlap [this message]
2019-03-26  0:43 ` [RESEND PATCH v6 00/12] x86: Enable FSGSBASE instructions Andy Lutomirski

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