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

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Message-ID:
 <TYCPR01MB8804C19B48AAFF1DBB1588498B0CA@TYCPR01MB8804.jpnprd01.prod.outlook.com>
Date: Mon, 8 Sep 2025 08:31:01 +0000
From: "Shaopeng Tan (Fujitsu)" <tan.shaopeng@...itsu.com>
To: 'Dave Martin' <Dave.Martin@....com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
CC: "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, James Morse <james.morse@....com>
Subject: RE: [PATCH] arm_mpam: Relax num_rmids parameter advertised to
 userspace

> 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.
> 
> Suggested-by: Shaopeng Tan <tan.shaopeng@...fujitsu.com>
> Signed-off-by: Dave Martin <Dave.Martin@....com>
> ---
> 
> This patch is based on:
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git
> mpam/snapshot/v6.16-rc5
> 
>  drivers/platform/arm64/mpam/mpam_resctrl.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/platform/arm64/mpam/mpam_resctrl.c
> b/drivers/platform/arm64/mpam/mpam_resctrl.c
> index f1e565cd1aac..f4f6fa9acfff 100644
> --- a/drivers/platform/arm64/mpam/mpam_resctrl.c
> +++ b/drivers/platform/arm64/mpam/mpam_resctrl.c
> @@ -1208,12 +1208,17 @@ static void mpam_resctrl_monitor_init(struct
> mpam_class *class,
>  		/*
>  		 * Unfortunately, num_rmid doesn't mean anything for
>  		 * mpam, and its exposed to user-space!
> -		 * num-rmid is supposed to mean the number of groups
> -		 * that can be created, both control or monitor groups.
> -		 * For mpam, each control group has its own pmg/rmid
> -		 * space.
> +		 *
> +		 * num-rmid is supposed to mean the minimum number of
> +		 * monitoring groups that can exist simultaneously, including
> +		 * the default monitoring group for each control group.
> +		 *
> +		 * For mpam, each control group has its own pmg/rmid space,
> so
> +		 * it is not appropriate to advertise the whole rmid_idx space
> +		 * here.  But the pmgs corresponding to the parent control
> +		 * group can be allocated freely:
>  		 */
> -		l3->num_rmid = 1;
> +		l3->num_rmid = mpam_pmg_max + 1;
> 
>  		switch (type) {
>  		case QOS_L3_MBM_LOCAL_EVENT_ID:
> 
> base-commit: b8e4905233fe45814b3c73be7e091f172cfb86ce
> --
> 2.34.1

Reviewed-by: Shaopeng Tan <tan.shaopeng@...fujitsu.com>

Thanks,
Shaopeng TAN