Re: [PATCH] fs/resctrl,x86/resctrl: Factor mba rounding to be per-arch

[Chen, Yu C]

On 9/30/2025 11:55 PM, Dave Martin wrote:
> Hi,
>
> On Tue, Sep 30, 2025 at 12:43:36PM +0800, Chen, Yu C wrote:
> > On 9/29/2025 10:13 PM, Dave Martin wrote:
>
> [...]
>
> > > I guess it would be up to the arch code whether to trust ACPI if it
> > > says that the maximum value of this field is > 511.  (> 65535 would be
> > > impossible though, since the fields would start to overlap each
> > > other...)
> > >
> > > Would anything break in the interface proposed here, if the maximum
> > > value is larger than 511?  (I'm hoping not.  For MPAM, the bandwidth
> > > controls can have up to 16 bits and the size can be probed though MMIO
> > > registers.
> >
> > I overlooked this bit width. It should not exceed 511 according to the
> > RDT spec. Previously, I was just wondering how to calculate the legacy
> > MB percentage in Tony's example. If we want to keep consistency - if
> > the user provides a value of 10, what is the denominator: Is it 255,
> > 511, or something queried from ACPI.
> >
> > MB: 0=4;1=75           <--- 10/255
> > #MB_HW: 0=10;1=191
> > #MB_MIN: 0=10;1=191
> > #MB_MAX: 0=64;1=191
> >
> > or
> >
> > MB: 0=1;1=75          <--- 10/511
> > #MB_HW: 0=10;1=191
> > #MB_MIN: 0=10;1=191
> > #MB_MAX: 0=64;1=191
> >
> > thanks,
> > Chenyu
>
> The denomiator (the "scale" parameter in my model, though the name is
> unimportant) should be whatever quantity of resource is specified in
> the "unit" parameter.
>
> For "percentage" type controls, I'd expect the unit to be 100% ("100pc"
> in my syntax).
>
> So, Tony suggestion looks plausible to me [1] :
>
>   | Yes. 255 (or whatever "Q" value is provided in the ACPI table)
>   | corresponds to no throttling, so 100% bandwidth.
>
> So, if ACPI says Q=387, that's the denominator we advertise.
>
> Does that sound right?

Yes, it makes sense, the denominator is the "scale" in your example.

> Question: is this a global parameter, or per-CPU?

It should be a global setting for all the MBA Register Blocks.

Thanks,
Chenyu

>  From the v1.2 RDT spec, it looks like it is a single, global parameter.
> I hope this is true (!)  But I'm not too familiar with these specs...
>
> Cheers
> ---Dave
>
>
> [1] https://lore.kernel.org/lkml/aNq11fmlac6dH4pH@agluck-desk3/
> >