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Date:   Tue, 18 May 2021 11:30:54 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Like Xu <like.xu@linux.intel.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        linux-kernel@vger.kernel.org,
        Kan Liang <kan.liang@linux.intel.com>,
        Borislav Petkov <bp@alien8.de>, seanjc@google.com,
        x86@kernel.org
Subject: Re: [PATCH v2 1/2] perf/x86: Skip checking if 0x0 MSR exists for
 guest Arch LBR
Message-ID: <YKOJTtmjvxbBJOcS@hirez.programming.kicks-ass.net>
References: <20210430052247.3079672-1-like.xu@linux.intel.com>
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On Fri, Apr 30, 2021 at 01:22:46PM +0800, Like Xu wrote:

> - Avoid checking unrelated Architecture MSR 0x0 in a simple way

I'm thinking the below is simpler still, no?

> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index 5272f349dca2..456aa6ffd9a1 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -6250,7 +6250,8 @@ __init int intel_pmu_init(void)
>  	 * Check all LBT MSR here.
>  	 * Disable LBR access if any LBR MSRs can not be accessed.
>  	 */
> -	if (x86_pmu.lbr_nr && !check_msr(x86_pmu.lbr_tos, 0x3UL))
> +	if (x86_pmu.lbr_nr && !static_cpu_has(X86_FEATURE_ARCH_LBR) &&
> +	    !check_msr(x86_pmu.lbr_tos, 0x3UL))
>  		x86_pmu.lbr_nr = 0;
>  	for (i = 0; i < x86_pmu.lbr_nr; i++) {
>  		if (!(check_msr(x86_pmu.lbr_from + i, 0xffffUL) &&

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index 2521d03de5e0..e28892270c58 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -6253,7 +6253,7 @@ __init int intel_pmu_init(void)
 	 * Check all LBT MSR here.
 	 * Disable LBR access if any LBR MSRs can not be accessed.
 	 */
-	if (x86_pmu.lbr_nr && !check_msr(x86_pmu.lbr_tos, 0x3UL))
+	if (x86_pmu.lbr_tos && !check_msr(x86_pmu.lbr_tos, 0x3UL))
 		x86_pmu.lbr_nr = 0;
 	for (i = 0; i < x86_pmu.lbr_nr; i++) {
 		if (!(check_msr(x86_pmu.lbr_from + i, 0xffffUL) &&