linux-kernel - Re: [RFC PATCH 13/38] arm_mpam: resctrl: Add CDP emulation

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Message-ID: <16b5063a-1201-4453-8038-4ef937d20abd@arm.com>
Date: Tue, 16 Dec 2025 14:24:46 +0000
From: Ben Horgan <ben.horgan@....com>
To: James Morse <james.morse@....com>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org
Cc: D Scott Phillips OS <scott@...amperecomputing.com>,
 carl@...amperecomputing.com, lcherian@...vell.com,
 bobo.shaobowang@...wei.com, tan.shaopeng@...itsu.com,
 baolin.wang@...ux.alibaba.com, Jamie Iles <quic_jiles@...cinc.com>,
 Xin Hao <xhao@...ux.alibaba.com>, peternewman@...gle.com,
 dfustini@...libre.com, amitsinght@...vell.com,
 David Hildenbrand <david@...nel.org>, Dave Martin <dave.martin@....com>,
 Koba Ko <kobak@...dia.com>, Shanker Donthineni <sdonthineni@...dia.com>,
 fenghuay@...dia.com, baisheng.gao@...soc.com,
 Jonathan Cameron <jonathan.cameron@...wei.com>, Gavin Shan
 <gshan@...hat.com>, rohit.mathew@....com, reinette.chatre@...el.com,
 Punit Agrawal <punit.agrawal@....qualcomm.com>
Subject: Re: [RFC PATCH 13/38] arm_mpam: resctrl: Add CDP emulation

Hi James,

On 12/5/25 21:58, James Morse wrote:
> Intel RDT's CDP feature allows the cache to use a different control value
> depending on whether the accesses was for instruction fetch or a data
> access. MPAM's equivalent feature is the other way up: the CPU assigns a
> different partid label to traffic depending on whether it was instruction
> fetch or a data access, which causes the cache to use a different control
> value based solely on the partid.
> 
> MPAM can emulate CDP, with the side effect that the alternative partid is
> seen by all MSC, it can't be enabled per-MSC.
> 
> Add the resctrl hooks to turn this on or off. Add the helpers that
> match a closid against a task, which need to be aware that the value
> written to hardware is not the same as the one resctrl is using.
> 
> Update the 'arm64_mpam_global_default' variable the arch code uses
> during context switch to know when the per-cpu value should be used
> instead.
> 
> Awkwardly, the MB controls don't implement CDP. To emulate this, the
> MPAM equivalent needs programming twice by the resctrl glue, as
> resctrl expects the bandwidth controls to be applied independently for
> both data and isntruction-fetch.
> 
> CC: Dave Martin <Dave.Martin@....com>
> CC: Ben Horgan <ben.horgan@....com>
> CC: Amit Singh Tomar <amitsinght@...vell.com>
> Signed-off-by: James Morse <james.morse@....com>
> ---
>  arch/arm64/include/asm/mpam.h  |   1 +
>  drivers/resctrl/mpam_resctrl.c | 115 ++++++++++++++++++++++++++++++++-
>  include/linux/arm_mpam.h       |   3 +
>  3 files changed, 118 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/mpam.h b/arch/arm64/include/asm/mpam.h
> index 2960ffaf6574..689285c4c61f 100644
> --- a/arch/arm64/include/asm/mpam.h
> +++ b/arch/arm64/include/asm/mpam.h
> @@ -4,6 +4,7 @@
>  #ifndef __ASM__MPAM_H
>  #define __ASM__MPAM_H
>  
> +#include <linux/arm_mpam.h>
>  #include <linux/bitops.h>
>  #include <linux/bitfield.h>
>  #include <linux/init.h>
> diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
> index c2650abb99ec..d5f75ed67e46 100644
> --- a/drivers/resctrl/mpam_resctrl.c
> +++ b/drivers/resctrl/mpam_resctrl.c
> @@ -33,6 +33,10 @@ static DEFINE_MUTEX(domain_list_lock);
>  static bool exposed_alloc_capable;
>  static bool exposed_mon_capable;
>  
> +/*
> + * MPAM emulates CDP by setting different PARTID in the I/D fields of MPAM0_EL1.
> + * This applies globally to all traffic the CPU generates.
> + */
>  static bool cdp_enabled;
>  
>  bool resctrl_arch_alloc_capable(void)
> @@ -45,6 +49,67 @@ bool resctrl_arch_mon_capable(void)
>  	return exposed_mon_capable;
>  }
>  
> +bool resctrl_arch_get_cdp_enabled(enum resctrl_res_level rid)
> +{
> +	switch (rid) {
> +	case RDT_RESOURCE_L2:
> +	case RDT_RESOURCE_L3:
> +		return cdp_enabled;
> +	case RDT_RESOURCE_MBA:
> +	default:
> +		/*
> +		 * x86's MBA control doesn't support CDP, so user-space doesn't
> +		 * expect it.
> +		 */
> +		return false;
> +	}
> +}
> +
> +/**
> + * resctrl_reset_task_closids() - Reset the PARTID/PMG values for all tasks.
> + *
> + * At boot, all existing tasks use partid zero for D and I.
> + * To enable/disable CDP emulation, all these tasks need relabelling.
> + */
> +static void resctrl_reset_task_closids(void)
> +{
> +	struct task_struct *p, *t;
> +
> +	read_lock(&tasklist_lock);
> +	for_each_process_thread(p, t) {
> +		resctrl_arch_set_closid_rmid(t, RESCTRL_RESERVED_CLOSID,
> +					     RESCTRL_RESERVED_RMID);
> +	}
> +	read_unlock(&tasklist_lock);
> +}
> +
> +int resctrl_arch_set_cdp_enabled(enum resctrl_res_level ignored, bool enable)
> +{
> +	u32 partid_i, partid_d;
> +
> +	cdp_enabled = enable;
> +	partid_i = partid_d = RESCTRL_RESERVED_CLOSID;
> +
> +	if (enable) {
> +		u32 partid = RESCTRL_RESERVED_CLOSID;
> +
> +		partid_d = resctrl_get_config_index(partid, CDP_CODE);
> +		partid_i = resctrl_get_config_index(partid, CDP_DATA);

These are back to front, partid_d is data and partid_i is code. Also, if
there is only one partid then we don't have 2 different partids and we
should fail to enable cdp.

> +	}
> +
> +	mpam_set_task_partid_pmg(current, partid_d, partid_i, 0, 0);
> +	WRITE_ONCE(arm64_mpam_global_default, mpam_get_regval(current));
> +
> +	resctrl_reset_task_closids();
> +
> +	return 0;
> +}

Thanks,

Ben