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

From: Borislav Petkov <bp@alien8.de>
To: Maxim Levitsky <mlevitsk@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
	Ingo Molnar <mingo@redhat.com>,
	Josh Poimboeuf <jpoimboe@kernel.org>,
	Namhyung Kim <namhyung@kernel.org>,
	Tony Luck <tony.luck@intel.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	"David S. Miller" <davem@davemloft.net>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"Chang S. Bae" <chang.seok.bae@intel.com>,
	Jane Malalane <jane.malalane@citrix.com>,
	Kees Cook <keescook@chromium.org>,
	Kan Liang <kan.liang@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, Herbert Xu <herbert@gondor.apana.org.au>,
	Jiri Olsa <jolsa@kernel.org>, Mark Rutland <mark.rutland@arm.com>,
	linux-perf-users@vger.kernel.org,
	"open list:CRYPTO API" <linux-crypto@vger.kernel.org>
Subject: Re: [PATCH v2 1/5] perf/x86/intel/lbr: use setup_clear_cpu_cap instead of clear_cpu_cap
Date: Mon, 19 Sep 2022 16:31:32 +0200	[thread overview]
Message-ID: <Yyh9RDbaRqUR1XSW@zn.tnic> (raw)
In-Reply-To: <20220718141123.136106-2-mlevitsk@redhat.com>

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.

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