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

[Dave Martin <Dave.Martin@arm.com>]

Hi there,

On Mon, Sep 29, 2025 at 05:19:32PM +0800, Chen, Yu C wrote:
> On 9/26/2025 6:58 AM, Luck, Tony wrote:

[...]

> > Applying this to Intel upcoming region aware memory bandwidth
> > that supports 255 steps and h/w min/max limits.
> > We would have info files with "min = 1, max = 255" and a schemata
> > file that looks like this to legacy apps:
> > 
> > MB: 0=50;1=75
> > #MB_HW: 0=128;1=191
> > #MB_MIN: 0=128;1=191
> > #MB_MAX: 0=128;1=191
> > 
> > But a newer app that is aware of the extensions can write:
> > 
> > # cat > schemata << 'EOF'
> > MB_HW: 0=10
> > MB_MIN: 0=10
> > MB_MAX: 0=64
> > EOF
> > 
> > which then reads back as:
> > MB: 0=4;1=75
> > #MB_HW: 0=10;1=191
> > #MB_MIN: 0=10;1=191
> > #MB_MAX: 0=64;1=191
> > 
> > with the legacy line updated with the rounded value of the MB_HW
> > supplied by the user. 10/255 = 3.921% ... so call it "4".
> > 
> 
> This seems to be applicable as it introduces the new interface
> while preserving forward compatibility.
> 
> One minor question is that, according to "Figure 6-5. MBA Optimal
> Bandwidth Register" in the latest RDT specification, the maximum
> value ranges from 1 to 511.
> Additionally, this bandwidth field is located at bits 48 to 56 in
> the MBA Optimal Bandwidth Register, and the range for
> this segment could be 1 to 8191. Just wonder if it would be
> possible that the current maximum value of 512 may be extended
> in the future? Perhaps we could explore a method to query the maximum upper
> limit from the ACPI table or register, or use CPUID to distinguish between
> platforms rather than hardcoding it. Reinette also mentioned this in another
> thread.
>
> Thanks,
> Chenyu
> 
> 
> [1] https://www.intel.com/content/www/us/en/content-details/851356/intel-resource-director-technology-intel-rdt-architecture-specification.html

I can't comment on the direction of travel in the RDT architecture.

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 don't think we've seen MPAM hardware that comes close to 16 bits for
now, though.

Cheers
---Dave