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

[Drew Fustini]

On Fri, Jun 05, 2026 at 12:35:51PM -0700, Drew Fustini wrote:
> > As I mentioned in response to Ben [2] there seems to be a mismatch between
> > architecture requirements here. resctrl uses the value returned by 
> > resctrl_get_default_ctrlval() as the control value that means "no throttling".
> > For Intel this means min == max but this does not seem to be the case for MPAM
> > and CBQRI. I am not familiar enough with either to have an alternative proposal here
> > so I need to become familiar now. There is a bit of backlog on other resctl
> > work right now so this will take me some time to sort out.
> 
> Thanks for pointing this out. In that case, it doesn't seem to match
> what I was thinking of for MB_MIN. The CBQRI reserved bandwidth blocks
> Rbwb) control can be thought of as a minimum amount of guranteed
> bandwidth for a control group. Each RCID (e.g. CLOSID) must be assigned
> at least 1 bandwidth block per the spec. Therefore, the membw.min_bw
> would need to be 1.
> 
> There is also a max bandwidth reservation across all control groups
> (RCIDs / CLOSIDs) so that there will be some amount of unreserved
> bandwidth. Mweight (1-255) controls how much of that unreserved
> bandwidth pool that a group can use. Mweight of 0 means no shared
> bandwidth. I think the membw.min_bw would need to 255 so that all groups
> get equal share of the unreserved pool.

Sorry, I wasn't thinking about this right. If Mweight is used for MB,
then membw.max_bw would be 100 (MAX_MBA_BW) and membw.min_bw would be 0
which means no shared bandwidth. 

> It seems like that would be incorrect use of membw.min_bw in both cases?

The issue is really just for Rbwb (reserved bandwidth) as that needs to
default to the minimum of 1. What about introducing membw.reset_val
which would be returned by resctrl_get_default_ctrl()?

MB could set membw.reset_val to be the same value as membw.max_bw.

> > > There is no equivalent to MB (percentage throttle) in RISC-V so I would
> > > want it to be valid to have MB_MIN (minimum reservation) without MB.
> > > 
> > > I rebased my RISC-V CBQRI v6 series on top of this proof of concept and
> > > was able to validate it works okay in Qemu:
> > > 
> > > MB_WGHT:72=255
> > >  MB_MIN:72=756
> > >      L2:64=fff;65=fff
> > >      L3:75=ffff
> > 
> > Ideally any new support should not break existing user space and the existing
> > user interface expects a MB entry in the schemata file when the MB resource exists.
> > Is it possible to emulate the percentage based MB control with MB_WGHT or MB_MIN?
> > This sounds similar as what is/was planned for MPAM [2].
> 
> Yes, I think that Mweight could be mapped to the MB concept of
> throttling. All groups could start with the max Mweight of 255 which
> could can be represented as 100%. 
> 
> However, I'm not sure what to do about membw.min_bw. Mweight = 0 means
> it can not use any of the shared unreserved bandwidth pool. If
> resctrl_get_default_ctrlval() is designed to mean "no throttling", then
> it seems like the membw.min_bw would need to be 255. But that feels
> weird for the min_bw value to be equal to the max weight for unreserved
> bandwidth.

MB would have the typical membw.min_bw = 100, and
resctrl_get_default_ctrl() would return 100. The controller would be
programmed with Mweight 255 for 100%.

Thanks,
Drew