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Received: from MN2PR12MB3053.namprd12.prod.outlook.com (2603:10b6:208:c7::24) by BYAPR12MB3607.namprd12.prod.outlook.com (2603:10b6:a03:dd::24) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4951.12; Fri, 4 Feb 2022 09:33:00 +0000 Received: from MN2PR12MB3053.namprd12.prod.outlook.com ([fe80::10c4:c928:bb18:44bc]) by MN2PR12MB3053.namprd12.prod.outlook.com ([fe80::10c4:c928:bb18:44bc%5]) with mapi id 15.20.4951.012; Fri, 4 Feb 2022 09:33:00 +0000 Message-ID: <9b890769-e769-83ed-c953-d25930b067ba@amd.com> Date: Fri, 4 Feb 2022 15:02:36 +0530 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.4.1 Subject: Re: [PATCH v2] perf/amd: Implement erratum #1292 workaround for F19h M00-0Fh Content-Language: en-US To: Jim Mattson Cc: like.xu.linux@gmail.com, eranian@google.com, santosh.shukla@amd.com, pbonzini@redhat.com, seanjc@google.com, wanpengli@tencent.com, vkuznets@redhat.com, joro@8bytes.org, peterz@infradead.org, mingo@redhat.com, alexander.shishkin@linux.intel.com, tglx@linutronix.de, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, kvm@vger.kernel.org, x86@kernel.org, linux-perf-users@vger.kernel.org, ananth.narayan@amd.com, kim.phillips@amd.com, Ravi Bangoria References: <2e96421f-44b5-c8b7-82f7-5a9a9040104b@amd.com> <20220202105158.7072-1-ravi.bangoria@amd.com> <3c97e081-ae46-d92d-fe8f-58642d6b773e@amd.com> From: Ravi Bangoria In-Reply-To: Content-Type: text/plain; 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Implement >>>> workaround for Zen3 Family 19 Model 00-0F processors as suggested in >>>> Revision Guide[1]: >>>> >>>> To count the non-FP affected PMC events correctly: >>>> o Use Core::X86::Msr::PERF_CTL2 to count the events, and >>>> o Program Core::X86::Msr::PERF_CTL2[43] to 1b, and >>>> o Program Core::X86::Msr::PERF_CTL2[20] to 0b. >>>> >>>> Note that the specified workaround applies only to counting events and >>>> not to sampling events. Thus sampling event will continue functioning >>>> as is. >>>> >>>> Although the issue exists on all previous Zen revisions, the workaround >>>> is different and thus not included in this patch. >>>> >>>> This patch needs Like's patch[2] to make it work on kvm guest. >>> >>> IIUC, this patch along with Like's patch actually breaks PMU >>> virtualization for a kvm guest. >>> >>> Suppose I have some code which counts event 0xC2 [Retired Branch >>> Instructions] on PMC0 and event 0xC4 [Retired Taken Branch >>> Instructions] on PMC1. I then divide PMC1 by PMC0 to see what >>> percentage of my branch instructions are taken. On hardware that >>> suffers from erratum 1292, both counters may overcount, but if the >>> inaccuracy is small, then my final result may still be fairly close to >>> reality. >>> >>> With these patches, if I run that same code in a kvm guest, it looks >>> like one of those events will be counted on PMC2 and the other won't >>> be counted at all. So, when I calculate the percentage of branch >>> instructions taken, I either get 0 or infinity. >> >> Events get multiplexed internally. See below quick test I ran inside >> guest. My host is running with my+Like's patch and guest is running >> with only my patch. > > Your guest may be multiplexing the counters. The guest I posited does not. It would be helpful if you can provide an example. > I hope that you are not saying that kvm's *thread-pinned* perf events > are not being multiplexed at the host level, because that completely > breaks PMU virtualization. IIUC, multiplexing happens inside the guest. Thanks, Ravi