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Date:   Wed, 21 Apr 2021 15:30:16 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Like Xu <like.xu@linux.intel.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Kan Liang <kan.liang@linux.intel.com>,
        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>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH RESEND 1/2] perf/x86: Skip checking MSR for MSR 0x0
Message-ID: <YIBFCOz9R966xAVZ@google.com>
References: <20210421021825.37872-1-like.xu@linux.intel.com>
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On Wed, Apr 21, 2021, Like Xu wrote:
> The Architecture LBR does not have MSR_LBR_TOS (0x000001c9).
> When ARCH_LBR we don't set lbr_tos, the failure from the
> check_msr() against MSR 0x000 will make x86_pmu.lbr_nr = 0,
> thereby preventing the initialization of the guest LBR.
> 
> Fixes: 47125db27e47 ("perf/x86/intel/lbr: Support Architectural LBR")
> Signed-off-by: Like Xu <like.xu@linux.intel.com>
> Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
> ---
>  arch/x86/events/intel/core.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index 5272f349dca2..5036496caa60 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -4751,10 +4751,10 @@ static bool check_msr(unsigned long msr, u64 mask)
>  	u64 val_old, val_new, val_tmp;
>  
>  	/*
> -	 * Disable the check for real HW, so we don't
> +	 * Disable the check for real HW or non-sense msr, so we don't

I think this should be "undefined MSR" or something along those lines.  MSR 0x0
is a "real" MSR, on Intel CPUs it's an alias for IA32_MC0_ADDR; at least it's
supposed to be, most/all Intel CPUs incorrectly alias it to IA32_MC0_CTL.

Anyways, my point is that if your definition of "nonsense" is any MSR that is
not a valid perf MSR, then this check is woefully incompletely.  If your
definition is a nonsensical value, then this comment is simply wrong.

What you're really looking for is precisely the case where the MSR was zero
initialized and never defined.

>  	 * mess with potentionaly enabled registers:
>  	 */
> -	if (!boot_cpu_has(X86_FEATURE_HYPERVISOR))
> +	if (!boot_cpu_has(X86_FEATURE_HYPERVISOR) || !msr)
>  		return true;
>  
>  	/*
> -- 
> 2.30.2
>