From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2B9A8C433EF for ; Mon, 18 Jul 2022 04:28:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232604AbiGREZl (ORCPT ); Mon, 18 Jul 2022 00:25:41 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45022 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229535AbiGREZj (ORCPT ); Mon, 18 Jul 2022 00:25:39 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 7346EE028 for ; Sun, 17 Jul 2022 21:25:38 -0700 (PDT) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 0CE751042; Sun, 17 Jul 2022 21:25:38 -0700 (PDT) Received: from [10.162.40.15] (unknown [10.162.40.15]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 0964E3F766; Sun, 17 Jul 2022 21:25:34 -0700 (PDT) Message-ID: <915cde39-7e3c-1a41-ca17-5953426fa0f3@arm.com> Date: Mon, 18 Jul 2022 09:55:32 +0530 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.1 Subject: Re: [PATCH v3] drivers/perf: arm_spe: Fix consistency of SYS_PMSCR_EL1.CX Content-Language: en-US To: Suzuki K Poulose , linux-arm-kernel@lists.infradead.org Cc: german.gomez@arm.com, james.clark@arm.com, Will Deacon , Mark Rutland , Alexey Budankov , linux-kernel@vger.kernel.org References: <20220714061302.2715102-1-anshuman.khandual@arm.com> From: Anshuman Khandual In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/14/22 16:40, Suzuki K Poulose wrote: > On 14/07/2022 07:13, Anshuman Khandual wrote: >> The arm_spe_pmu driver will enable SYS_PMSCR_EL1.CX in order to add CONTEXT >> packets into the traces, if the owner of the perf event runs with required >> capabilities i.e CAP_PERFMON or CAP_SYS_ADMIN via perfmon_capable() helper. >> >> The value of this bit is computed in the arm_spe_event_to_pmscr() function >> but the check for capabilities happens in the pmu event init callback i.e >> arm_spe_pmu_event_init(). This suggests that the value of the CX bit should >> remain consistent for the duration of the perf session. >> >> However, the function arm_spe_event_to_pmscr() may be called later during >> the event start callback i.e arm_spe_pmu_start() when the "current" process >> is not the owner of the perf session, hence the CX bit setting is currently >> not consistent. >> >> One way to fix this, is by caching the required value of the CX bit during >> the initialization of the PMU event, so that it remains consistent for the >> duration of the session. It uses currently unused 'event->hw.flags' element >> to cache perfmon_capable() value, which can be referred during event start >> callback to compute SYS_PMSCR_EL1.CX. This ensures consistent availability >> of context packets in the trace as per event owner capabilities. >> >> Drop BIT(SYS_PMSCR_EL1_CX_SHIFT) check in arm_spe_pmu_event_init(), because >> now CX bit cannot be set in arm_spe_event_to_pmscr() with perfmon_capable() >> disabled. >> >> Cc: Will Deacon >> Cc: Mark Rutland >> Cc: Alexey Budankov >> Cc: linux-arm-kernel@lists.infradead.org >> Cc: linux-kernel@vger.kernel.org >> Fixes: cea7d0d4a59b ("drivers/perf: Open access for CAP_PERFMON privileged process") > > TBH, this is not sufficient. The above commit simply replaced the capable() check with perfmon_capable() wrapper. The "incorrect check > in the wrong task context" existed since : > > d5d9696b0380 ("drivers/perf: Add support for ARMv8.2 Statistical Profiling Extension"). > > So I would recommend using that for the Fixes tag. And any stable > backports without perfmon_capable() could fallback to using capable() > check, like we do in this patch, from the event_init. Makes sense. I will respin the patch with above "Fixes: " tag unless there is some other feedback here. > > Otherwise, > > Reviewed-by: Suzuki K Poulose >