Re: [PATCH v6 21/40] arm_mpam: resctrl: Hide CDP emulation behind CONFIG_EXPERT

[Zeng Heng]

Hi Ben,

On 2026/3/13 22:45, Ben Horgan wrote:
> When CDP is not enabled, the 'rmid_entry's in the limbo list,
> rmid_busy_llc, map directly to a (PARTID,PMG) pair and when CDP is enabled
> the mapping is to two different pairs. As the limbo list is reused between
> mounts and CDP disabled on unmount this can lead to stale mapping and the
> limbo handler will then make monitor reads with potentially out of range
> PARTID. This may then cause an MPAM error interrupt and the driver will
> disable MPAM.
>
> No problems are expected if you just mount the resctrl file system
> once with CDP enabled and never unmount it. Hide CDP emulation behind
> CONFIG_EXPERT to protect the unwary.
>
> Signed-off-by: Ben Horgan <ben.horgan@xxxxxxx>
> ---
> Adding this ugliness in the hope of avoiding patch churn and extra
> reviewer work. I am looking into the resctrl changes needed to fix this.
> ---
>   drivers/resctrl/mpam_resctrl.c | 12 ++++++++++++
>   1 file changed, 12 insertions(+)
>
> diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
> index 903d1a0f564f..cab3e9ccb5c7 100644
> --- a/drivers/resctrl/mpam_resctrl.c
> +++ b/drivers/resctrl/mpam_resctrl.c
> @@ -82,6 +82,18 @@ int resctrl_arch_set_cdp_enabled(enum resctrl_res_level rid, bool enable)
>   	u32 partid_i = RESCTRL_RESERVED_CLOSID, partid_d = RESCTRL_RESERVED_CLOSID;
>   	int cpu;
>   
> +	if (!IS_ENABLED(CONFIG_EXPERT) && enable) {
> +		/*
> +		 * If the resctrl fs is mounted more than once, sequentially,
> +		 * then CDP can lead to the use of out of range PARTIDs.
> +		 */
> +		pr_warn("CDP not supported\n");
> +		return -EOPNOTSUPP;
> +	}
> +
> +	if (enable)
> +		pr_warn("CDP is an expert feature and may cause MPAM to malfunction.\n");
> +

Agree that a proper fix will require modifications to resctrl to handle 
the limbo state across mount sessions. Hiding CDP behind CONFIG_EXPERT 
is acceptable as a short-term mitigation to prevent users from 
accidentally hitting the mentioned issue.

Reviewed-by: Zeng Heng <zengheng4@xxxxxxxxxx>