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

Jonathan Cameron
2025-12-18T11:58:21Z

On Fri, 5 Dec 2025 21:58:36 +0000
James Morse <james.morse@arm.com> 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>
More trivial stuff. It is clearly that kind of a day.

> 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

> +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;

I'd do these at the declarations. The kernel style guide does also
say not to do multiple assignments on one line. Obviously this
one is fairly obviously fine but none the less...

> +
> + 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);
> + }
> +
> + 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;
> +}

> @@ -272,6 +369,7 @@ u32 resctrl_arch_get_config(struct rdt_resource *r, struct rdt_ctrl_domain *d,
> int resctrl_arch_update_one(struct rdt_resource *r, struct rdt_ctrl_domain *d,
> u32 closid, enum resctrl_conf_type t, u32 cfg_val)
> {
> + int err;
> u32 partid;
> struct mpam_config cfg;
> struct mpam_props *cprops;
> @@ -311,7 +409,22 @@ int resctrl_arch_update_one(struct rdt_resource *r, struct rdt_ctrl_domain *d,
> return -EINVAL;
> }
>
> - return mpam_apply_config(dom->ctrl_comp, partid, &cfg);
> + /*
> + * When CDP is enabled, but the resource doesn't support it, we need to
> + * apply the same configuration to the other partid.
> + */
> + if (mpam_resctrl_hide_cdp(r->rid)) {
> + partid = resctrl_get_config_index(closid, CDP_CODE);
> + err = mpam_apply_config(dom->ctrl_comp, partid, &cfg);
> + if (err)
> + return err;
> +
> + partid = resctrl_get_config_index(closid, CDP_DATA);
> + return mpam_apply_config(dom->ctrl_comp, partid, &cfg);

Given you returned, maybe drop the else which will reduce the diff
as well as a bonus. Definitely drop this blank line.

> +
> + } else {
> + return mpam_apply_config(dom->ctrl_comp, partid, &cfg);
> + }
> }