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b=td8U3cDGsWXiaP6mUav5iWtqZZvGeaUY5BBKypdGUqT6EFpjCldxfDr77Q0TXkBGkSElRwJT4qiyTxVwx9eJDIeR4oHUHQ9Xokqr5K2U9vJ0XjkHTv9N/OSUR5CyVlAL2UCF3GBuoX612WhXvoPFOj1rN7wZr3gOgeJ7MtGryd8= Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=amd.com; Received: from DS0PR12MB6584.namprd12.prod.outlook.com (2603:10b6:8:d0::6) by DM4PR12MB7672.namprd12.prod.outlook.com (2603:10b6:8:103::18) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.8356.14; Fri, 17 Jan 2025 04:51:37 +0000 Received: from DS0PR12MB6584.namprd12.prod.outlook.com ([fe80::45e0:b675:7986:3305]) by DS0PR12MB6584.namprd12.prod.outlook.com ([fe80::45e0:b675:7986:3305%4]) with mapi id 15.20.8356.010; Fri, 17 Jan 2025 04:51:37 +0000 Message-ID: <337e17e7-b3db-44a1-9493-495b4c7313eb@amd.com> Date: Fri, 17 Jan 2025 10:21:28 +0530 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 5/9] perf/amd/ibs: Don't allow freq mode event creation through ->config interface To: Peter Zijlstra Cc: mingo@redhat.com, namhyung@kernel.org, acme@kernel.org, eranian@google.com, irogers@google.com, kan.liang@linux.intel.com, bp@alien8.de, x86@kernel.org, linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, santosh.shukla@amd.com, ananth.narayan@amd.com, sandipan.das@amd.com, Ravi Bangoria References: <20250115054438.1021-1-ravi.bangoria@amd.com> <20250115054438.1021-6-ravi.bangoria@amd.com> <20250116210959.GB7232@noisy.programming.kicks-ass.net> Content-Language: en-US From: Ravi Bangoria In-Reply-To: <20250116210959.GB7232@noisy.programming.kicks-ass.net> Content-Type: text/plain; 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Since the sample period is programmed in these control registers, >> IBS PMU driver allows opening an IBS event by setting sample period >> value directly in perf_event_attr->config instead of using explicit >> perf_event_attr->sample_period interface. >> >> However, this logic is not applicable for freq mode events since the >> semantics of control register fields are applicable only to fixed >> sample period whereas the freq mode event adjusts sample period after >> each and every sample. Currently, IBS driver (unintentionally) allows >> creating freq mode event via ->config interface, which is semantically >> wrong as well as detrimental because it can be misused to bypass >> perf_event_max_sample_rate checks. >> >> Don't allow freq mode event creation through perf_event_attr->config >> interface. > > So this is the case where: > > perf_event_attr = { > .freq = 1. > .sample_freq = 0, > } > > Right, where we would then take the period from the .config fields? > > So that seems sensible enough; but does this not break existing users? > Or are we lucky enough to not have any users of this borken behaviour, > unlike the earlier broken behaviour that we now have to live with? I'm not aware of any existing tools using the ->config interface. Also, I can't think of any advantage of the ->config interface over stranded one. So, if anyone is using it and start seeing an error, they can switch to { attr->freq=1, attr->sample_freq=xxx } and there is no functional breakage. Thanks for the review, Ravi