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 9DCB2C433FE for ; Fri, 18 Nov 2022 06:15:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241090AbiKRGO6 (ORCPT ); Fri, 18 Nov 2022 01:14:58 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39902 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240866AbiKRGOy (ORCPT ); Fri, 18 Nov 2022 01:14:54 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id BBA8E43AE2; Thu, 17 Nov 2022 22:14:52 -0800 (PST) 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 CE39823A; Thu, 17 Nov 2022 22:14:58 -0800 (PST) Received: from [10.162.41.7] (unknown [10.162.41.7]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 2EDD13F73B; Thu, 17 Nov 2022 22:14:47 -0800 (PST) Message-ID: <7e5d7724-153d-15c3-1f78-002980b67499@arm.com> Date: Fri, 18 Nov 2022 11:44:44 +0530 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.2.2 Subject: Re: [PATCH V5 6/7] arm64/perf: Add BRBE driver Content-Language: en-US To: James Clark Cc: Mark Brown , Rob Herring , Marc Zyngier , Suzuki Poulose , Ingo Molnar , linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, linux-arm-kernel@lists.infradead.org, peterz@infradead.org, acme@kernel.org, mark.rutland@arm.com, will@kernel.org, catalin.marinas@arm.com References: <20221107062514.2851047-1-anshuman.khandual@arm.com> <20221107062514.2851047-7-anshuman.khandual@arm.com> <1d06f05c-0e7f-7648-6b1b-b78e999bc65f@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 11/17/22 15:39, James Clark wrote: > > > On 17/11/2022 05:45, Anshuman Khandual wrote: >> >> >> On 11/16/22 22:12, James Clark wrote: >>> >>> >>> On 07/11/2022 06:25, Anshuman Khandual wrote: >>> [...] >>> >>>> +static void perf_branch_to_brbcr(struct pmu_hw_events *cpuc, int branch_type) >>>> +{ >>>> + cpuc->brbcr = (BRBCR_EL1_CC | BRBCR_EL1_MPRED); >>>> + >>>> + if (branch_type & PERF_SAMPLE_BRANCH_USER) >>>> + cpuc->brbcr |= BRBCR_EL1_E0BRE; >>>> + >>>> + if (branch_type & PERF_SAMPLE_BRANCH_NO_CYCLES) >>>> + cpuc->brbcr &= ~BRBCR_EL1_CC; >>>> + >>>> + if (branch_type & PERF_SAMPLE_BRANCH_NO_FLAGS) >>>> + cpuc->brbcr &= ~BRBCR_EL1_MPRED; >>>> + >>>> + if (branch_type & PERF_SAMPLE_BRANCH_KERNEL) >>>> + cpuc->brbcr |= BRBCR_EL1_E1BRE; >>>> + else >>>> + return; >>>> + >>>> + /* >>>> + * The exception and exception return branches could be >>>> + * captured only when the event has necessary privilege >>>> + * indicated via branch type PERF_SAMPLE_BRANCH_KERNEL, >>>> + * which has been ascertained in generic perf. Please >>>> + * refer perf_copy_attr() for more details. >>>> + */ >>>> + if (branch_type & PERF_SAMPLE_BRANCH_ANY) { >>>> + cpuc->brbcr |= BRBCR_EL1_EXCEPTION; >>>> + cpuc->brbcr |= BRBCR_EL1_ERTN; >>> >>> Because this comes after the PERF_SAMPLE_BRANCH_KERNEL check, it's >>> impossible to get syscall records from userspace. When you enable kernel >>> branch records, the buffer always fills up before it gets to userspace. >> >> Just to summerize. >> >> System call [user_addr -> kernel_addr] and return [kernel_addr -> user_addr] >> records are impossible to be captured, because >> >> - Without PERF_SAMPLE_BRANCH_KERNEL, BRBCR_EL1_EXCEPTION/ERTN are not set >> - With PERF_SAMPLE_BRANCH_KERNEL, buffer fills up with in kernel branches >> > > Yep that's it > >> Did you try with latest fix, that clears the paused BRBE after reading branch >> records during PMU interrupt ? That fix creates much more samples than before. >> > > Yes that's with the latest fix. It may even make the problem more > obvious with the fix rather than without. Okay. > >>> >>> Can you move this to the top so that it can be set if either >>> PERF_SAMPLE_BRANCH_USER or PERF_SAMPLE_BRANCH_KERNEL is set. The >> >> Why should they depend on privilege filters i.e PERF_SAMPLE_BRANCH_USER/KERNEL >> rather than just branch filters PERF_SAMPLE_BRANCH_ANY/ANY_CALL/ANY_RETURN ? >> > > Exactly, I don't think they should depend on the privilege level. But at > the moment we return before setting them unless > PERF_SAMPLE_BRANCH_KERNEL is set. Okay. > >>> hardware already handles the security by giving partial records with the >>> kernel part zeroed out so I don't think the driver needs to add any >>> additional rules other than setting BRBCR_EL1_E1BRE or BRBCR_EL1_E0BRE. >> >> Basically BRBCR_EL1_EXCEPTION/BRBCR_EL1_ERTN should be treated like any other >> branch filter rather than privilege filters as is the case now ? > > I think so yes > >> >>> >>> For example I moved it to the top, removed the return below and then I >>> get syscall partial records: >>> >>> .... 5: 0000000000745d0c -> 0000000000000000 0 cycles P 9fbfbfbf SYSCALL >>> >>> I also get ERETS but with only the userspace part set: >>> >>> ..... 4: 0000000000000000 -> 0000000000745d10 0 cycles P 9fbfbfbf ERET >> But with both user and kernel privilege filters being set, these should have >> been complete branch records containing both user and kernel addresses ? > > Yes, but I only set PERF_SAMPLE_BRANCH_USER, I should have given the > perf command as well: > > perf record -j any,save_type,u -- syscall_loop > > Where syscall_loop obviously generates lots of SYSCALLS and ERETS. But > with both user and kernel you just don't get to that point before the > buffer fills up. At least in per process mode, maybe with -a the timings > are different. Fair enough, will change BRBCR_EL1_EXCEPTION/BRBCR_EL1_ERTN as discussed.