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

[Andy Lutomirski]

On Fri, Apr 10, 2015 at 8:50 AM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxx>
> ---
>  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. */
> +#define ELF_HWCAP2             kernel_enabled_features;

Could we name this something more like elf_hwcap2 instead of
kernel_enabled_features?

--Andy
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