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

[Chen, Yu C]

Hi Reinette,

On 6/10/2026 11:59 PM, Reinette Chatre wrote:
> Hi Chenyu,

[ ... ]

> > Regarding the region-aware RDT case, I wonder if we actually need to emulate the
> > legacy MB control using MB_MAX. First, when we refer to the "legacy" for region-aware
> > RDT, I suppose it corresponds to "MSR access" plus "percentage-based control".
> >
> > case 1:
> > If the platform does not support region-aware RDT (no ERDT table is detected),
> > the MB is naturally the "legacy" MB, and the info directory would look like:
> >
> > info
> > └── MB
> >          └── resource_schemata
> >                  └── MB
> >
> > case 2:If the platform supports region-aware RDT (i.e., ERDT parsing succeeds),
> > then the structure looks like below:
> >
> > info
> > └── MB
> >          └── resource_schema
> >                  └── MB                  <=== legacy
> >                  └── MB_REGION0_OPT
> >                  └── MB_REGION1_OPT
> >                  └── MB_REGION0_MIN
> >                  └── MB_REGION1_MIX
> >                  └── MB_REGION0_MAX
> >                  └── MB_REGION1_MAX
> >
> > The OS supports runtime switching between the legacy MB and
> > region-aware MB, via a MB scope control like:
> > /sys/fs/resctrl/info/MB/resource_schemata/mode
> > [legacy] native
> > If the user wants to use the legacy interface, the system
> > will fall back to the default [MSR-based, percentage]; if the
> > user wants to use the new interface, the region-aware interfaces
> > will be used. Since the two modes are mutually exclusive, there
> > seems to be no need to use MB_REGION0_MAX to emulate legacy MB.
>
> -  This implies that all current and future ERDT capable systems will keep
>     supporting MSR access with the legacy "percentage based control". Is this
>     accurate? 

I don't know the answer for now; it may depend on how the hardware 
maintains
backward compatibility. Let me have a check.

> What will behavior be on an ERDT system that does not support
>     MSR access with the percentage-based control?

Then in this case, we may need to emulate the legacy MBA if users do
not wish to switch to the new interface.

> -  It is not obvious to me what you plan to have as default interface -
>     I believe this should be "legacy" interface to ensure this will not
>     break user space. resctrl should only switch to "native" after user
>     demonstrates (by switching) familiarity with the new interface.

Yes, setting "legacy" as the default will accommodate users who rely on
the old interfaces.

> Reinette