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

[Chen, Yu C]

Hi Reinette,

On 6/11/2026 11:45 PM, Reinette Chatre wrote:
> Hi Chenyu,
>
> On 6/10/26 8:26 PM, Chen, Yu C wrote:
> > On 6/10/2026 11:59 PM, Reinette Chatre wrote:
> > [ ... ]
> >
> > > -  This implies that all current and future ERDT capable systems will keep
> > >      supporting MSR access with the legacy "percentage based control". Is this
> > >      accurate? What will behavior be on an ERDT system that does not support
> > >      MSR access with the percentage-based control?
> >
> > Would it make sense to encourage users to switch to MMIO-based access if the
> > platform replaces MSR interfaces with MMIO-based ones? One issue with emulating
> > MSR-based access is that there may not be a strict 1:1 mapping between the MSR
> > range and the MMIO range. Additionally, I am unsure whether both are strictly
> > linearly scalable, so it could be challenging to create a proper mapping for
> > msr_value_emulated = map(mmio_value) IIUC.
>
> resctrl needs to continue to offer the MB percentage based control to not break any
> existing tools that may take a while to transition to the new control interface.
>
> I believe all architectures will encourage their users to use the finer grained
> controls while I also expect users and their tools would gradually switch by themselves
> to benefit from the improved capabilities.

The following are current thoughts on how Control B can emulate Control A.
May I know if this direction is acceptable?

The main idea is that, Legacy MBA controller delegates to region-aware MBA
controller via a pointer, with its hw_update function converting 
percentages
to the target's value range.

struct resctrl_hw_ctrl {
	struct resctrl_ctrl  r_ctrl;
	unsigned int         msr_base;
	void                 (*hw_update)(struct hw_param *m);
	/* points to the backing controller */
	struct resctrl_ctrl  *r_ctrl_emul;  <-- newly added
};

On boot, region-aware controllers shall be created first, followed by the
legacy MBA controller. When ERDT is enabled for legacy MBA, hw_update is
assigned to mba_emul_update() instead of mba_wrmsr_intel(). Meanwhile,
r_ctrl_emul points to the MAX controller of region0.

In mba_emul_update(), the legacy MSR value is scaled to the range of 
region0
MAX controller by referencing r_ctrl_emul->membw.max_bw and 
r_ctrl_emul->membw.min_bw.
This maps the original input range [1, 100] to a new range [1, L], where 
L <= 255.

This emulation logic can also be extended to other controllers.

thanks,
Chenyu