linux-kernel - Re: [PATCH v4 RESEND 4/5] perf/x86/lbr: Skip checking for the existence of LBR

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Message-ID: <20210323214935.GF4746@worktop.programming.kicks-ass.net>
Date:   Tue, 23 Mar 2021 22:49:35 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Like Xu <like.xu@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Kan Liang <kan.liang@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v4 RESEND 4/5] perf/x86/lbr: Skip checking for the
 existence of LBR_TOS for Arch LBR

On Mon, Mar 22, 2021 at 02:06:34PM +0800, Like Xu wrote:
> The Architecture LBR does not have MSR_LBR_TOS (0x000001c9). KVM will
> generate #GP for this MSR access, thereby preventing the initialization
> of the guest LBR.
> 
> Fixes: 47125db27e47 ("perf/x86/intel/lbr: Support Architectural LBR")
> Signed-off-by: Like Xu <like.xu@...ux.intel.com>
> Reviewed-by: Kan Liang <kan.liang@...ux.intel.com>
> Reviewed-by: Andi Kleen <ak@...ux.intel.com>
> ---
>  arch/x86/events/intel/core.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index 382dd3994463..7f6d748421f2 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -5740,7 +5740,8 @@ __init int intel_pmu_init(void)
>  	 * Check all LBR 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 && !boot_cpu_has(X86_FEATURE_ARCH_LBR) &&
> +	    !check_msr(x86_pmu.lbr_tos, 0x3UL))
>  		x86_pmu.lbr_nr = 0;

But when ARCH_LBR we don't set lbr_tos, so we check MSR 0x000, not 0x1c9.

Do we want check_msr() to ignore msr==0 ?
Additionally, do we want a check for lbr_info ?

>  	for (i = 0; i < x86_pmu.lbr_nr; i++) {
>  		if (!(check_msr(x86_pmu.lbr_from + i, 0xffffUL) &&
> -- 
> 2.29.2
>