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

Ben Horgan
2025-12-16T14:24:51Z

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@arm.com>
> CC: Ben Horgan <ben.horgan@arm.com>
> CC: Amit Singh Tomar <amitsinght@marvell.com>
> Signed-off-by: James Morse <james.morse@arm.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