Re: [PATCH v2 1/5] perf/x86/intel/lbr: use setup_clear_cpu_cap instead of clear_cpu_cap

Borislav Petkov
2022-09-19T14:31:42Z

On Mon, Jul 18, 2022 at 05:11:19PM +0300, Maxim Levitsky wrote:
> clear_cpu_cap(&boot_cpu_data) is very similar to setup_clear_cpu_cap
> except that the latter also sets a bit in 'cpu_caps_cleared' which
> later clears the same cap in secondary cpus, which is likely
> what is meant here.
>
> Fixes: 47125db27e47 ("perf/x86/intel/lbr: Support Architectural LBR")
>
> Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
> arch/x86/events/intel/lbr.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
> index 13179f31fe10fa..b08715172309a7 100644
> --- a/arch/x86/events/intel/lbr.c
> +++ b/arch/x86/events/intel/lbr.c
> @@ -1860,7 +1860,7 @@ void __init intel_pmu_arch_lbr_init(void)
> return;
>
> clear_arch_lbr:
> - clear_cpu_cap(&boot_cpu_data, X86_FEATURE_ARCH_LBR);
> + setup_clear_cpu_cap(X86_FEATURE_ARCH_LBR);

setup_clear_cpu_cap() has a very specific purpose - see
apply_forced_caps().

This whole call sequence is an early_initcall() which is way after the
whole CPU features picking apart happens.

So what is actually this fixing?

--
Regards/Gruss,
Boris.