linux-kernel - Re: [PATCH] arm_mpam: Relax num_rmids parameter advertised to userspace

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Message-ID: <58eae4c3-51b3-48ad-b077-aefdff969119@arm.com>
Date: Wed, 27 Aug 2025 18:16:00 +0100
From: James Morse <james.morse@....com>
To: Dave Martin <Dave.Martin@....com>, linux-kernel@...r.kernel.org
Cc: linux-arm-kernel@...ts.infradead.org,
 Shaopeng Tan <tan.shaopeng@...fujitsu.com>
Subject: Re: [PATCH] arm_mpam: Relax num_rmids parameter advertised to
 userspace

Hi Dave,

On 20/08/2025 14:15, Dave Martin wrote:
> On MPAM systems, monitoring groups are identified in the hardware by a
> (PARTID, PMG) pair.  Two monitoring group identifiers are the same only
> if the PARTIDs and PMGs both match.  This means that the number of
> monitoring groups that can be created in each control group is the same
> as the number of distinct PMG values supported by the hardware.  The
> number of monitoring groups that exist in other control groups at the
> same time makes no difference to this.
> 
> Currently, the MPAM driver takes the cautious approach and always
> num_rmids = 1.
> 
> Relax this limit, by advertising the number of distinct PMG values
> supported by the hardware.
> 
> Code/Data Prioritization (CDP) makes no difference, since although this
> doubles the number of (PARTID, PMG) pairs available to a control group,
> each monitoring group now consumes two pairs instead of one.

Thanks, I've picked this up. It may get squashed into the patch that adds these lines
before going upstream...


Thanks,

James