linux-kernel - Re: [PATCH v2 01/15] perf/x86/lbr: Simplify the exposure check for the LBR

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Date:   Thu, 22 Dec 2022 17:41:24 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, kan.liang@...ux.intel.com,
        wei.w.wang@...el.com, Like Xu <like.xu@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        "Paolo Bonzini - Distinguished Engineer (kernel-recipes.org) (KVM HoF)" 
        <pbonzini@...hat.com>, Jim Mattson <jmattson@...gle.com>,
        kvm list <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Yang Weijiang <weijiang.yang@...el.com>
Subject: Re: [PATCH v2 01/15] perf/x86/lbr: Simplify the exposure check for
 the LBR_INFO registers

On Thu, Dec 22, 2022, Like Xu wrote:
> Hi Peter, would you help apply this one in your tip/perf tree,
> as it doesn't seem to be closely tied to the KVM changes. Thanks.
> 
> On 25/11/2022 12:05 pm, Yang Weijiang wrote:
> > From: Like Xu <like.xu@...ux.intel.com>
> > 
> > The x86_pmu.lbr_info is 0 unless explicitly initialized, so there's
> > no point checking x86_pmu.intel_cap.lbr_format.
> > 
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Reviewed-by: Kan Liang <kan.liang@...ux.intel.com>
> > Reviewed-by: Andi Kleen <ak@...ux.intel.com>
> > Signed-off-by: Like Xu <like.xu@...ux.intel.com>
> > Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> > ---
> >   arch/x86/events/intel/lbr.c | 4 +---
> >   1 file changed, 1 insertion(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
> > index 4dbde69c423b..e7caabfa1377 100644
> > --- a/arch/x86/events/intel/lbr.c
> > +++ b/arch/x86/events/intel/lbr.c
> > @@ -1606,12 +1606,10 @@ void __init intel_pmu_arch_lbr_init(void)
> >    */
> >   void x86_perf_get_lbr(struct x86_pmu_lbr *lbr)
> >   {
> > -	int lbr_fmt = x86_pmu.intel_cap.lbr_format;
> > -
> >   	lbr->nr = x86_pmu.lbr_nr;
> >   	lbr->from = x86_pmu.lbr_from;
> >   	lbr->to = x86_pmu.lbr_to;
> > -	lbr->info = (lbr_fmt == LBR_FORMAT_INFO) ? x86_pmu.lbr_info : 0;
> > +	lbr->info = x86_pmu.lbr_info;

This stable-worthy a bug fix, no?  E.g. won't the existing code misreport lbr->info
if the format is LBR_FORMAT_INFO2?

> >   }
> >   EXPORT_SYMBOL_GPL(x86_perf_get_lbr);