linux-kernel - Re: [RFC V1 06/11] arm64/perf: Drive BRBE from perf event states

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Date:   Wed, 26 Jan 2022 11:07:24 -0600
From:   Rob Herring <robh@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-perf-users@...r.kernel.org
Subject: Re: [RFC V1 06/11] arm64/perf: Drive BRBE from perf event states

On Mon, Jan 24, 2022 at 10:00:48AM +0530, Anshuman Khandual wrote:
> Branch stack sampling rides along the normal perf event and all the branch
> records get captured during the PMU interrupt. This just changes perf event
> handling on the arm64 platform to accommodate required BRBE operations that
> will enable branch stack sampling support.
> 
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: linux-perf-users@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: linux-arm-kernel@...ts.infradead.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> ---
>  arch/arm64/kernel/perf_event.c |  6 +++++
>  drivers/perf/arm_pmu.c         | 40 ++++++++++++++++++++++++++++++++++
>  2 files changed, 46 insertions(+)
> 
> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
> index f6a47036b0b4..11c82c8f2eec 100644
> --- a/arch/arm64/kernel/perf_event.c
> +++ b/arch/arm64/kernel/perf_event.c
> @@ -864,6 +864,12 @@ static irqreturn_t armv8pmu_handle_irq(struct arm_pmu *cpu_pmu)
>  		if (!armpmu_event_set_period(event))
>  			continue;
>  
> +		if (has_branch_stack(event)) {
> +			cpu_pmu->brbe_read(cpuc, event);

Is has_branch_stack() guaranteed to be false on arm32? If not, this will 
be a NULL function ptr. 

To add to my other comments, this patch is where I would add 
brbe_read(), etc. to arm_pmu.

Rob