Re: [PATCH V2 0/7] arm64/perf: Enable branch stack sampling
From: Anshuman Khandual <hidden>
Date: 2022-09-13 12:13:05
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On 9/13/22 16:25, James Clark wrote:
On 08/09/2022 06:10, Anshuman Khandual wrote:quoted
This series enables perf branch stack sampling support on arm64 platform via a new arch feature called Branch Record Buffer Extension (BRBE). All relevant register definitions could be accessed here. https://developer.arm.com/documentation/ddi0601/2021-12/AArch64-Registers This series applies on v6.0-rc4 after the BRBE related perf ABI changes series (V7) that was posted earlier, and a branch sample filter helper patch. https://lore.kernel.org/all/20220824044822.70230-1-anshuman.khandual@arm.com/ (local) https://lore.kernel.org/all/20220906084414.396220-1-anshuman.khandual@arm.com/ (local) Following issues have been resolved - Jame's concerns regarding permission inadequacy related to perfmon_capable() - Jame's concerns regarding using perf_event_paranoid along with perfmon_capable()I don't see the resolution to this one. I'm not 100% sure of the code path used for LBR, but I think you just need to take perf_allow_kernel() into account somewhere to make this command have the same result with BRBE. Is there any contention that the permissions shouldn't behave in the same way across platforms? This is when perf_event_paranoid < 2: Intel: $ perf record -j any -- ls [ perf record: Woken up 1 times to write data ] [ perf record: Captured and wrote 0.014 MB perf.data (16 samples) ] Arm: $ perf record -j any -- ls Error: No permission to enable cycles event.
Proposed solution here just follows what we did for the SPE driver recently. I would not be surprised, if there is difference in semantics in permission checking across various platform perf drivers. Ideally permission should not even be checked in platform drivers - either capability or perf_event_paranoid. Unfortunately changing the permission checking framework across generic perf is beyond the scope for this BRBE proposal and might be taken up later via a different series. Although I would be willing to accommodate any alternate suggestions to improve permission checking here in the BRBE driver. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel