Re: [PATCH 5/5] x86, perf: Avoid context switching LBR_INFO when not needed

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Oct 20, 2015 at 11:46:37AM -0700, Andi Kleen wrote:
> +++ b/arch/x86/kernel/cpu/perf_event_intel.c
> @@ -2844,7 +2844,7 @@ static void intel_pmu_cpu_starting(int cpu)
>  	/*
>  	 * Deal with CPUs that don't clear their LBRs on power-up.
>  	 */
> -	intel_pmu_lbr_reset();
> +	intel_pmu_lbr_reset(1);

s/1/true/ ?


> @@ -242,7 +245,7 @@ static void __intel_pmu_lbr_restore(struct x86_perf_task_context *task_ctx)
>  
>  	if (task_ctx->lbr_callstack_users == 0 ||
>  	    task_ctx->lbr_stack_state == LBR_NONE) {
> -		intel_pmu_lbr_reset();
> +		intel_pmu_lbr_reset(task_ctx->need_info > 0);
>  		return;
>  	}
>
> @@ -317,7 +320,7 @@ void intel_pmu_lbr_sched_task(struct perf_event_context *ctx, bool sched_in)
>  	 * stack with branch from multiple tasks.
>   	 */
>  	if (sched_in) {
> -		intel_pmu_lbr_reset();
> +		intel_pmu_lbr_reset(!task_ctx || task_ctx->need_info > 0);
>  		cpuc->lbr_context = ctx;
>  	}
>  }
> @@ -340,7 +343,7 @@ void intel_pmu_lbr_enable(struct perf_event *event)
>  	 * avoid data leaks.
>  	 */
>  	if (event->ctx->task && cpuc->lbr_context != event->ctx) {
> -		intel_pmu_lbr_reset();
> +		intel_pmu_lbr_reset(!(event->hw.branch_reg.reg & LBR_NO_INFO));
>  		cpuc->lbr_context = event->ctx;
>  	}
>  	cpuc->br_sel = event->hw.branch_reg.reg;

Are you sure none of that will result in some data leak in a weird corner
case?