Re: [PATCH 1/5] x86/asm: Clarify static_cpu_has()'s intended use

Peter Zijlstra
2019-04-01T08:06:30Z

On Sat, Mar 30, 2019 at 12:20:18PM +0100, Borislav Petkov wrote:
> From: Borislav Petkov <bp@suse.de>
>
> Clarify when one should use static_cpu_has() and when one should use
> boot_cpu_has().
>
> Requested-by: Nadav Amit <nadav.amit@gmail.com>
> Signed-off-by: Borislav Petkov <bp@suse.de>
> Cc: x86@kernel.org
> ---
> arch/x86/include/asm/cpufeature.h | 9 ++++++---
> 1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
> index 30cf12c81db3..1d337c51f7e6 100644
> --- a/arch/x86/include/asm/cpufeature.h
> +++ b/arch/x86/include/asm/cpufeature.h
> @@ -156,9 +156,12 @@ extern void clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int bit);
> #else
>
> /*
> - * Static testing of CPU features. Used the same as boot_cpu_has().
> - * These will statically patch the target code for additional
> - * performance.
> + * Static testing of CPU features. Used the same as boot_cpu_has(). It
> + * statically patches the target code for additional performance. Use
> + * static_cpu_has() only in fast paths, where every cycle counts. Which
> + * means that the boot_cpu_has() variant is already fast enough for the
> + * majority of cases and you should stick to using it as it is generally
> + * only two instructions: a RIP-relative MOV and a TEST.
> */
> static __always_inline bool _static_cpu_has(u16 bit)
> {

Should we introduce cpu_has() ?

I'm sure it boot_cpu_has() is an awesome name, but in like 99.9% of the
cases we don't give a crap about which actual CPU has the feature set. We
also don't actually support asymmetric cpu features anyway.