Re: [PATCH 6/8] x86: Enumerate kernel FSGS capability in AT_HWCAP2

[Borislav Petkov <bp@alien8.de>]

On Fri, Apr 10, 2015 at 08:50:31AM -0700, Andi Kleen wrote:
> From: Andi Kleen <ak@linux.intel.com>
> 
> The kernel needs to explicitely enable RD/WRFSBASE to handle context
> switch correctly. So the application needs to know if it can safely use
> these instruction. Just looking at the CPUID bit is not enough because it
> may be running in a kernel that does not enable the instructions.
> 
> One way for the application would be to just try and catch the SIGILL.
> But that is difficult to do in libraries which may not want
> to overwrite the signal handlers of the main application.
> 
> So we need to provide a way for the application to discover the kernel
> capability.
> 
> I used AT_HWCAP2 in the ELF aux vector which is already used by
> PPC for similar things. We define a new Linux defined bitmap
> returned in AT_HWCAP.  Currently it has only one bit set,
> for kernel is FSGSBASE capable.
> 
> The application can then access it manually or using
> the getauxval() function in newer glibc.
> 
> Signed-off-by: Andi Kleen <ak@linux.intel.com>
> ---
>  arch/x86/include/asm/elf.h        | 7 +++++++
>  arch/x86/include/uapi/asm/hwcap.h | 7 +++++++
>  arch/x86/kernel/cpu/common.c      | 7 ++++++-
>  3 files changed, 20 insertions(+), 1 deletion(-)
>  create mode 100644 arch/x86/include/uapi/asm/hwcap.h
> 
> diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
> index ca3347a..47dac31 100644
> --- a/arch/x86/include/asm/elf.h
> +++ b/arch/x86/include/asm/elf.h
> @@ -257,6 +257,13 @@ extern int force_personality32;
>  
>  #define ELF_HWCAP		(boot_cpu_data.x86_capability[0])
>  
> +extern unsigned kernel_enabled_features;
> +
> +/* HWCAP2 supplies kernel enabled CPU feature, so that the application
> +   can know that it can safely use them. The bits are defined in
> +   uapi/asm/hwcap.h. */

Comments formatting.

> +#define ELF_HWCAP2		kernel_enabled_features;
> +
>  /* This yields a string that ld.so will use to load implementation
>     specific libraries for optimization.  This is more specific in
>     intent than poking at uname or /proc/cpuinfo.
> diff --git a/arch/x86/include/uapi/asm/hwcap.h b/arch/x86/include/uapi/asm/hwcap.h
> new file mode 100644
> index 0000000..d9c54f8
> --- /dev/null
> +++ b/arch/x86/include/uapi/asm/hwcap.h
> @@ -0,0 +1,7 @@
> +#ifndef _ASM_HWCAP_H
> +#define _ASM_HWCAP_H 1
> +
> +#define HWCAP2_FSGSBASE	(1 << 0) 	/* Kernel enabled RD/WR FS/GS BASE */

BIT()

> +/* upto bit 31 free */
> +
> +#endif
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 7df88a3..81186cb 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -34,6 +34,7 @@
>  #include <asm/i387.h>
>  #include <asm/fpu-internal.h>
>  #include <asm/mtrr.h>
> +#include <asm/hwcap.h>
>  #include <linux/numa.h>
>  #include <asm/asm.h>
>  #include <asm/cpu.h>
> @@ -49,6 +50,8 @@
>  
>  #include "cpu.h"
>  
> +unsigned kernel_enabled_features __read_mostly;
> +
>  /* all of these masks are initialized in setup_cpu_local_masks() */
>  cpumask_var_t cpu_initialized_mask;
>  cpumask_var_t cpu_callout_mask;
> @@ -958,8 +961,10 @@ static void identify_cpu(struct cpuinfo_x86 *c)
>  	numa_add_cpu(smp_processor_id());
>  #endif
>  
> -	if (cpu_has(c, X86_FEATURE_FSGSBASE))
> +	if (cpu_has(c, X86_FEATURE_FSGSBASE)) {

static_cpu_has_safe()

-- 
Regards/Gruss,
    Boris.

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