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

[Chen, Yu C]

Hi Reinette,

On 8/11/2026 2:09 AM, Reinette Chatre wrote:
> Hi Chenyu,
> > I think "legacy" now refers to "MSR" in the context of region-aware RDT,
> > due to lack of mapping between MSR-based and MMIO-based values.
>
>  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?

Yes, agreed.

> > Then for the current region-aware MBA, should we add region-aware controllers
> > as the same level as legacy controller, instead of adding them in legacy
> > controllers' emulated_by list, since the legacy controller is not
> > emulated by region-aware controllers? And we might need to introduce enable/disable
> > flag per controller, which could be switched on/off when switching between
> > legacy and native.
> Please consider what will be required to support per-controller dynamic ("enable" vs "disable")
> state: resctrl fs needs to expose interfaces that accept control changes from user space after
> initialization that needs to be supported by architecture. In the current design resctrl fs expects
> controllers to be set and configured at the time it starts so such change would require new locking
> between resctrl fs and architecture. This complexity does not seem necessary to me since switching
> between legacy and native accomplishes the same, no?

OK, agreed.

> I find "region-aware controllers at the same level as legacy controller" together with
> "per controller enable/disable" conflicting since it creates impression that user space can
> enable/disable the controllers independently while there is a required dependency among the
> different controllers based on whether hardware has region-aware MBA enabled or not. Even
> here there would be a requirement to communicate to userspace which controllers could be
> enabled/disabled together ... which is the same as the emulated list, no?

I see. So we actually need a single option (control_mode) to switch 
between modes (legacy vs.
native, mutually exclusive), rather than giving each controller its own 
independent enable/disable
toggle. The emulated list is one way to implement control_mode, ensuring 
"legacy" and "native"
are mutually exclusive.

> Also consider the future:
> (a) Would it be possible to map legacy control value to region aware control value in the future?
>      I believe the answer is "yes". If resctrl starts by displaying the region-aware controllers
>      as emulating the legacy controller in info/ then the transition to support this will
>      be smooth.
>
> (b) Would MSR interface always be available?
>      I do not think resctrl should always assume this to be the case. Here I also find that starting
>      with exposing region aware controllers as emulating the legacy controller in info/ to support
>      a smooth transition.
>
> What problem(s) do you envision?

Based on this design, I don’t see any issues. So putting native 
controllers under the legacy
controller could be a viable approach that can be extended in the future.

thanks,
Chenyu