Re: [PATCH v3 11/47] arm64: mpam: Add helpers to change a task or cpu's MPAM PARTID/PMG values

Gavin Shan
2026-01-19T07:01:58Z

Hi Ben,

On 1/13/26 12:58 AM, Ben Horgan wrote:
> From: James Morse <james.morse@arm.com>
>
> Care must be taken when modifying the PARTID and PMG of a task in any
> per-task structure as writing these values may race with the task being
> scheduled in, and reading the modified values.
>
> Add helpers to set the task properties, and the CPU default value. These
> use WRITE_ONCE() that pairs with the READ_ONCE() in mpam_get_regval() to
> avoid causing torn values.
>
> CC: Dave Martin <Dave.Martin@arm.com>
> Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
> Signed-off-by: James Morse <james.morse@arm.com>
> Signed-off-by: Ben Horgan <ben.horgan@arm.com>
> ---
> Changes since rfc:
> Keep comment attached to mpam_get_regval()
> Add internal helper, __mpam_regval() (Jonathan)
> ---
> arch/arm64/include/asm/mpam.h | 28 ++++++++++++++++++++++++++++
> 1 file changed, 28 insertions(+)
>
> diff --git a/arch/arm64/include/asm/mpam.h b/arch/arm64/include/asm/mpam.h
> index 7b3d3abad162..c9b73f1af7ce 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/bitfield.h>
> #include <linux/jump_label.h>
> #include <linux/percpu.h>
> #include <linux/sched.h>
> @@ -22,6 +23,22 @@ DECLARE_PER_CPU(u64, arm64_mpam_current);
> */
> extern u64 arm64_mpam_global_default;
>
> +static inline u64 __mpam_regval(u16 partid_d, u16 partid_i, u8 pmg_d, u8 pmg_i)
> +{
> + return FIELD_PREP(MPAM0_EL1_PARTID_D, partid_d) |
> + FIELD_PREP(MPAM0_EL1_PARTID_I, partid_i) |
> + FIELD_PREP(MPAM0_EL1_PMG_D, pmg_d) |
> + FIELD_PREP(MPAM0_EL1_PMG_I, pmg_i);
> +}

Nitpick: Alignment issues in the lines for 2nd/3rd/4th FIELD_PREP().

return FIELD_PREP(...) |
FIELD_PREP(...) |
FIELD_PREP(...) |
FIELD_PREP(...);

> +
> +static inline void mpam_set_cpu_defaults(int cpu, u16 partid_d, u16 partid_i,
> + u8 pmg_d, u8 pmg_i)
> +{
> + u64 default_val = __mpam_regval(partid_d, partid_i, pmg_d, pmg_i);
> +
> + WRITE_ONCE(per_cpu(arm64_mpam_default, cpu), default_val);
> +}
> +

per_cpu(arm64_mpam_default) won't be reachable until CONFIG_ARM64_MPAM is set.
So I think both mpam_set_cpu_defaults() and __mpam_regval() need to be protected
by '#ifdef CONFIG_ARM64_MPAM ... #endif'.

> /*
> * The resctrl filesystem writes to the partid/pmg values for threads and CPUs,
> * which may race with reads in mpam_thread_switch(). Ensure only one of the old
> @@ -36,6 +53,17 @@ static inline u64 mpam_get_regval(struct task_struct *tsk)
> return READ_ONCE(task_thread_info(tsk)->mpam_partid_pmg);
> }
>
> +static inline void mpam_set_task_partid_pmg(struct task_struct *tsk,
> + u16 partid_d, u16 partid_i,
> + u8 pmg_d, u8 pmg_i)
> +{
> +#ifdef CONFIG_ARM64_MPAM
> + u64 regval = __mpam_regval(partid_d, partid_i, pmg_d, pmg_i);
> +
> + WRITE_ONCE(task_thread_info(tsk)->mpam_partid_pmg, regval);
> +#endif
> +}
> +
> static inline void mpam_thread_switch(struct task_struct *tsk)
> {
> u64 oldregval;

Thanks,
Gavin