Messages in this thread | | | Date | Mon, 1 Apr 2019 10:06:23 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 1/5] x86/asm: Clarify static_cpu_has()'s intended use |
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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.
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