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X-OriginatorOrg: intel.com Hi Reinette, On 9/19/2026 7:11 AM, Reinette Chatre wrote: > Hi Chenyu, > >> On Mon, Aug 10, 2026 at 11:09:31AM -0700, Reinette Chatre wrote: >>> >>> From user space view "legacy" is the existing percentage based MB control >>> that user space has been using until now. The only way to support this "legacy" >>> user interface with region-aware hardware is to use the MSR interface, no? >>> User space does not care whether the hardware uses MSR or MMIO, it just uses >>> the MB interface. >>> That is, as you also say, until it is possible to map legacy control value to >>> region-aware control value at which point the MMIO interface can be used and >>> user space can continue to use the MB control without interruption. >>> >> >> While looking at the context of control_mode under the info directory, it is >> currently used only for the controller to switch between "legacy" and "native". >> However, there is also an explicit requirement for the MBM to switch between >> "legacy" and "native" TOGETHER with the MBA. >> >> According to the RDT spec 6.1.3.1 RDT Control Register for CPU Agents, >> it discourages mixing legacy MSR for MBM with region aware MBA: >> "It is recommended that software use Region Aware MBM when Region Aware >> MBA is enabled and vice versa. Mixed mode use (e.g, legacy MSR >> interfaces for MBM with Region Aware MBA or vice versa) is not >> supported and may lead to inconsistent behavior" >> >> That is to say, I'm trying to add logic in the code so that, if control_mode >> has switched, the corresponding "mode" for MBM will also be adjusted to the >> same mode. The code can enable/create both legacy MBM events/sysfs and region-aware >> MBM events/sysfs during bootup, and make one of them visible according to control_mode, >> similar to what Babu does in PLZA when hiding a file in >> https://lore.kernel.org/lkml/f7bcf19ac8113a1e3d9575739c1a6400c455ce6d.1787772750.git.babu.moger@amd.com/ >> >> May I know whether this approach is doable? > This sounds reasonable to me. Since so many changes are between current resctrl and those > changes it is difficult to envision how clean such change would be. We should aim to > avoid sprinkling "if ("region aware") then" checks all over the place. > > A switch like this will force "region aware" to support the same number of CLOS/RMID > as the MSR interface. Is this a concern? > It seems that taking the weakest link (minimum) across all sources is a pervasive convention in resctrl, which can be used to avoid out-of-bounds CLOS/RMID access. If a future platform does not support the CPUID/MSR interfaces, we can only rely on the ACPI table to get the max RMID/CLOSID. thanks, Chenyu