On Tue, Oct 20, 2015 at 11:46:33AM -0700, Andi Kleen wrote:
> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>
> This fixes a bug added with the earlier 90405aa02. The bug
> could lead to lost LBR call stacks. When restoring the LBR
> state we need to use the TOS of the previous context, not
> the current context. To do that we need to save/restore the tos.
Current best practise also asks for:
Fixes: 90405aa02247 ("perf/x86/intel/lbr: Limit LBR accesses to TOS in callstack mode")
> Cc: <stable@xxxxxxxxxxxxxxx> # 4.2+
> Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> --- a/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> @@ -239,7 +239,7 @@ static void __intel_pmu_lbr_restore(struct x86_perf_task_context *task_ctx)
> }
>
> mask = x86_pmu.lbr_nr - 1;
> - tos = intel_pmu_lbr_tos();
> + tos = task_ctx->tos;
> for (i = 0; i < tos; i++) {
> lbr_idx = (tos - i) & mask;
> wrmsrl(x86_pmu.lbr_from + lbr_idx, task_ctx->lbr_from[i]);
> @@ -247,6 +247,7 @@ static void __intel_pmu_lbr_restore(struct x86_perf_task_context *task_ctx)
> if (x86_pmu.intel_cap.lbr_format == LBR_FORMAT_INFO)
> wrmsrl(MSR_LBR_INFO_0 + lbr_idx, task_ctx->lbr_info[i]);
> }
> + wrmsrl(x86_pmu.lbr_tos, tos);
> task_ctx->lbr_stack_state = LBR_NONE;
> }
Any idea who much more expensive that wrmsr() is compared to the rdmsr()
it replaces?
If its significant we could think about having this behaviour depend on
callstacks.
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