linux-kernel - Re: [PATCH v2 1/5] perf/x86/intel/lbr: use setup_clear_cpu_cap instead of clear_cpu

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Date:   Mon, 19 Sep 2022 16:31:32 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        "David S. Miller" <davem@...emloft.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Jane Malalane <jane.malalane@...rix.com>,
        Kees Cook <keescook@...omium.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Jiri Olsa <jolsa@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-perf-users@...r.kernel.org,
        "open list:CRYPTO API" <linux-crypto@...r.kernel.org>
Subject: Re: [PATCH v2 1/5] perf/x86/intel/lbr: use setup_clear_cpu_cap
 instead of clear_cpu_cap

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@...ux.intel.com>
> Signed-off-by: Maxim Levitsky <mlevitsk@...hat.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.

https://people.kernel.org/tglx/notes-about-netiquette