Re: [RFC v2] arm,x86,fs/resctrl: Generic schema description Proof of Concept

["Chen, Yu C" <yu.c.chen@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