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Received: from BL1PR12MB5320.namprd12.prod.outlook.com (2603:10b6:208:314::17) by CH3PR12MB7523.namprd12.prod.outlook.com (2603:10b6:610:148::13) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.592.12; Mon, 9 Nov 2026 15:33:14 +0000 Received: from BL1PR12MB5320.namprd12.prod.outlook.com ([fe80::1876:4a6d:2cf5:b8d1]) by BL1PR12MB5320.namprd12.prod.outlook.com ([fe80::1876:4a6d:2cf5:b8d1%5]) with mapi id 15.21.0592.012; Mon, 9 Nov 2026 15:33:13 +0000 Message-ID: <94e2852c-e002-44cc-b3e1-ccde8e92ccf3@amd.com> Date: Mon, 9 Nov 2026 09:33:11 -0600 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 4/5] selftests/resctrl: Use bandwidth values for AMD MBA schemata To: Reinette Chatre , Swapnil Sapkal , tony.luck@intel.com, fenghua.yu@intel.com, shuah@kernel.org Cc: Dave.Martin@arm.com, james.morse@arm.com, peternewman@google.com, eranian@google.com, tan.shaopeng@fujitsu.com, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, x86@kernel.org References: <20261022132517.46805-1-swapnil.sapkal@amd.com> <20261022132517.46805-5-swapnil.sapkal@amd.com> <4f656f59-9f2c-4774-9d9b-3a4d7b7b509d@intel.com> <39598840-29b1-4524-806f-9ef9dc2a3944@amd.com> <857d915f-9439-408b-9109-82143bd96ace@intel.com> Content-Language: en-US From: Babu Moger In-Reply-To: <857d915f-9439-408b-9109-82143bd96ace@intel.com> Content-Type: text/plain; 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The >>>>>> + * test sweeps the bandwidth from 4 GB/s to 40 GB/s in steps of 4 GB/s, >>>>>> + * which the hardware represents in units of 1/8 GB/s (i.e. 4 GB/s -> 32, >>>>>> + * 8 GB/s -> 64, ... 40 GB/s -> 320). >>>>>> + */ >>>>>> +#define AMD_ALLOCATION_MAX    320 >>>>> >>>>> The AMD maximum is enumerated from hardware. How can it be ensured this is >>>>> the appropriate maximum for all AMD systems? >>>>> >>>> >>>> As Babu mentioned, it's difficult for a single-threaded workload to >>>> reach the hardware-reported value. For that reason, I used these >>>> values to verify whether throttling takes effect. >>> >>> Just to confirm, my question was not about needing this value to be the actual >>> maximum of a system. Instead, I just wanted to confirm that this "320" value that >>> is hardcoded here for all AMD systems is a _valid_ value. Since the actual maximum >>> value is enumerated from  hardware it is not obvious to me that all systems will support >>> this hardcoded maximum. >>> >> >> The value 320 corresponds to 40 GB/s. I have tested this on several systems and was able to achieve the expected 40 GB/s bandwidth. The tests did not fail. >> >> There are some older systems where the test is not expected to run because AMD UMC support is not available. >> >> If you are concerned, since we are no longer depending on the maximum value, we can be conservative and start with the following adjusted values: >> >> #define AMD_ALLOCATION_MIN    32 >> #define AMD_ALLOCATION_STEP    16 >> >> This allows the value to reach 32 + 16 × 10(num_allocs) = 192, which corresponds to approximately 24 GB/s. >> >> Hope this answers your question. >> > I seem to have trouble articulating my question. Let me try to ask a different way. My bad. > > Consider the scenario where: > #define AMD_ALLOCATION_MAX 320 > > If the below (kernel code) snippet is run on all AMD systems on which the > test could run: > u32 eax, ebx, ecx, edx; > cpuid_count(0x80000020, 1, &eax, &ebx, &ecx, &edx); > > Is it possible for eax to ever contain 8 or less? > No. It has never been below 11 (0xB), so this should be fine. Thanks Babu