linux-kernel - Re: [PATCH 2/2] powerpc/perf: Fix mmcra corruption by bhrb

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Date:   Sat, 11 May 2019 08:17:21 +0530
From:   Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
To:     peterz@...radead.org, jolsa@...hat.com, mpe@...erman.id.au,
        maddy@...ux.vnet.ibm.com
Cc:     acme@...nel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
Subject: Re: [PATCH 2/2] powerpc/perf: Fix mmcra corruption by bhrb_filter



On 5/11/19 8:12 AM, Ravi Bangoria wrote:
> Consider a scenario where user creates two events:
> 
>   1st event:
>     attr.sample_type |= PERF_SAMPLE_BRANCH_STACK;
>     attr.branch_sample_type = PERF_SAMPLE_BRANCH_ANY;
>     fd = perf_event_open(attr, 0, 1, -1, 0);
> 
>   This sets cpuhw->bhrb_filter to 0 and returns valid fd.
> 
>   2nd event:
>     attr.sample_type |= PERF_SAMPLE_BRANCH_STACK;
>     attr.branch_sample_type = PERF_SAMPLE_BRANCH_CALL;
>     fd = perf_event_open(attr, 0, 1, -1, 0);
> 
>   It overrides cpuhw->bhrb_filter to -1 and returns with error.
> 
> Now if power_pmu_enable() gets called by any path other than
> power_pmu_add(), ppmu->config_bhrb(-1) will set mmcra to -1.
> 
> Signed-off-by: Ravi Bangoria <ravi.bangoria@...ux.ibm.com>

Fixes: 3925f46bb590 ("powerpc/perf: Enable branch stack sampling framework")