Re: [PATCH v2] x86,fs/resctrl: Factor MBA parse-time conversion to be per-arch

Ben Horgan
2025-11-10T11:06:36Z

Hi Dave,

On 10/31/25 15:41, Dave Martin wrote:
> The control value parser for the MB resource currently coerces the
> memory bandwidth percentage value from userspace to be an exact
> multiple of the rdt_resource::resctrl_membw::bw_gran parameter.
>
> On MPAM systems, this results in somewhat worse-than-worst-case
> rounding, since the bandwidth granularity advertised to resctrl by the
> MPAM driver is in general only an approximation to the actual hardware
> granularity on these systems, and the hardware bandwidth allocation
> control value is not natively a percentage -- necessitating a further
> conversion in the resctrl_arch_update_domains() path, regardless of the
> conversion done at parse time.
>
> Allow the arch to provide its own parse-time conversion that is
> appropriate for the hardware, and move the existing conversion to x86.
> This will avoid accumulated error from rounding the value twice on MPAM
> systems.
>
> Clarify the documentation, but avoid overly exact promises.
>
> Clamping to bw_min and bw_max still feels generic: leave it in the core
> code, for now.
>
> No functional change.
>
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
>
Seems sensible and helpful for MPAM.

Reviewed-by: Ben Horgan <ben.horgan@arm.com>

Thanks,

Ben