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Tue, 30 Sep 2025 04:43:45 +0000 Message-ID: <541fb538-0c22-4ad2-9c6e-83ada7f2240b@intel.com> Date: Tue, 30 Sep 2025 12:43:36 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] fs/resctrl,x86/resctrl: Factor mba rounding to be per-arch To: Dave Martin , "Luck, Tony" CC: , Reinette Chatre , James Morse , "Thomas Gleixner" , Ingo Molnar , "Borislav Petkov" , Dave Hansen , "H. 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MBA Optimal >> Bandwidth Register" in the latest RDT specification, the maximum >> value ranges from 1 to 511. >> Additionally, this bandwidth field is located at bits 48 to 56 in >> the MBA Optimal Bandwidth Register, and the range for >> this segment could be 1 to 8191. Just wonder if it would be >> possible that the current maximum value of 512 may be extended >> in the future? Perhaps we could explore a method to query the maximum upper >> limit from the ACPI table or register, or use CPUID to distinguish between >> platforms rather than hardcoding it. Reinette also mentioned this in another >> thread. >> >> Thanks, >> Chenyu >> >> >> [1] https://www.intel.com/content/www/us/en/content-details/851356/intel-resource-director-technology-intel-rdt-architecture-specification.html > > I can't comment on the direction of travel in the RDT architecture. > > I guess it would be up to the arch code whether to trust ACPI if it > says that the maximum value of this field is > 511. (> 65535 would be > impossible though, since the fields would start to overlap each > other...) > > Would anything break in the interface proposed here, if the maximum > value is larger than 511? (I'm hoping not. For MPAM, the bandwidth > controls can have up to 16 bits and the size can be probed though MMIO > registers. > I overlooked this bit width. It should not exceed 511 according to the RDT spec. Previously, I was just wondering how to calculate the legacy MB percentage in Tony's example. If we want to keep consistency - if the user provides a value of 10, what is the denominator: Is it 255, 511, or something queried from ACPI. MB: 0=4;1=75 <--- 10/255 #MB_HW: 0=10;1=191 #MB_MIN: 0=10;1=191 #MB_MAX: 0=64;1=191 or MB: 0=1;1=75 <--- 10/511 #MB_HW: 0=10;1=191 #MB_MIN: 0=10;1=191 #MB_MAX: 0=64;1=191 thanks, Chenyu > I don't think we've seen MPAM hardware that comes close to 16 bits for > now, though. > > Cheers > ---Dave