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Thu, 25 Jun 2026 14:59:14 +0000 Message-ID: <6ca0a35a-7dd4-4d79-878b-3d01026ff5df@intel.com> Date: Thu, 25 Jun 2026 22:59:04 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 1/6] x86/resctrl: Parse ACPI ERDT table and map RMDD domains by L3 cache ID To: Reinette Chatre CC: "x86@kernel.org" , "linux-kernel@vger.kernel.org" , "tglx@kernel.org" , "bp@alien8.de" , "mingo@redhat.com" , "dave.hansen@linux.intel.com" , "hpa@zytor.com" , "dave.martin@arm.com" , "james.morse@arm.com" , "fenghuay@nvidia.com" , "babu.moger@amd.com" , "Keshavamurthy, Anil" , "chen.yu@linux.dev" , "Luck, Tony" References: <807a91c2b6235f85e2d19d4293c17e53a8cc69b2.1781332698.git.yu.c.chen@intel.com> <288153df-307a-40e3-96e1-ff4ae8f6bc5a@intel.com> <2835922f-d2b7-45dc-92e3-c6aed4e79fe9@intel.com> <4b48ec4d-517d-44d0-8be0-7e24d0970b7b@intel.com> <1c671ba2-501d-4b31-bd33-9cb23b54db01@intel.com> <2152d065-c978-4ac3-a75c-358a763a4394@intel.com> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: <2152d065-c978-4ac3-a75c-358a763a4394@intel.com> Content-Type: text/plain; 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As new domains are created during CPU online their >>> initialization can just point to the needed data, no? >> >> Reinette, >> >> Historical note. The xarray was added during early development on a simulator where the L3 >> domain numbers were wildly sparse. That turned out to be an artifact of the simulator >> CPUID and BIOS interactions. On real h/w the L3 domains show up as 0, 1, 2, 3, ... > > resctrl dynamically allocates monitoring domains as the CPUs come online and the monitoring > domains are assigned the domain ID discovered from the system. This should not be impacted by > sparse domain numbers and works without an xarray, no? What am I missing? > >> But the xarray does still serve some purpose. Without it you’d need two passes through the >> ACPI ERDT structure. One to count how many RMDD substructures are present to allocate >> space, then second pass to parse things. > The data structures containing the domain data discovered from ERDT could just be dynamically > allocated with a single pass as the domains are discovered, similar to how rdt_resource::mon_domains > is populated, no? The difference is that the "ERDT domains" are discovered during initialization. > > I do not see a need for a separate xarray indexed by domain ID during "runtime" since we already > have the architecture specific domain structures. struct rdt_l3_mon_domain could just point to > its associated ERDT domain when it (struct rdt_l3_mon_domain) is initialized, no? > > You could use an xarray during initial RMDD parsing but from what I understand its index > is of no use since it was established early in this discussion that RMDD domain ID cannot be > assumed to be the same as enumerated via CPUID leaf 0x4. So even if you use an xarray the > CPU online handler needs to iterate over the xarray instead of indexing directly into it > to find CPU being onlined and through that determine the "RMDD domain ID" associated with the > CPUID leaf 0x4 cache ID of CPU being onlined. What is the benefit of an xarray when never > indexing into it? > If erdt_domain_info is embedded within rdt_hw_l3_mon_domain, an xarray might be not mandatory - any data structure form works; we only need to store the data in erdt.c for later lookup in the CPU online handler. If erdt_domain_info is maintained separately as in current version, we need to create mappings from L3 IDs to erdt_domain_info inside erdt_mon_read(). The xarray serves as a fast lookup mechanism when sparse arrays are not used(waste memory). thanks, Chenyu