Re: [RFC PATCH 05/38] arm64: mpam: Add helpers to change a task or cpu's MPAM PARTID/PMG values

Ben Horgan
2025-12-19T11:56:28Z

Hi Jonathan,

On 12/18/25 10:44, Jonathan Cameron wrote:
> On Fri, 5 Dec 2025 21:58:28 +0000
> James Morse <james.morse@arm.com> wrote:
>
>> 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>
>> CC: Ben Horgan <ben.horgan@arm.com>
>> Signed-off-by: James Morse <james.morse@arm.com>
>> ---
>> arch/arm64/include/asm/mpam.h | 30 ++++++++++++++++++++++++++++++
>> 1 file changed, 30 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/mpam.h b/arch/arm64/include/asm/mpam.h
>> index 86a55176f884..2960ffaf6574 100644
>> --- a/arch/arm64/include/asm/mpam.h
>> +++ b/arch/arm64/include/asm/mpam.h
>> @@ -5,6 +5,7 @@
>> #define __ASM__MPAM_H
>>
>> #include <linux/bitops.h>
>> +#include <linux/bitfield.h>
>> #include <linux/init.h>
>> #include <linux/jump_label.h>
>> #include <linux/percpu.h>
>> @@ -37,6 +38,35 @@ extern u64 arm64_mpam_global_default;
>> * A value in struct thread_info is used instead of struct task_struct as the
>> * cpu's u64 register format is used, but struct task_struct has two u32'.
>> */
> I'd be tempted to reorder this so the comment ends up near at least
> one use of task_thread_info(tsk)->mpam_partid_pmg. Otherwise it is even
> less obvious what it is referring to.
>
> Easiest might be to put the mpam_get_regval() before the mpam_set_task_partid_pmg()

Yes, this comment positioning was confusing.

>
>
>> +static inline void mpam_set_cpu_defaults(int cpu, u16 partid_d, u16 partid_i,
>> + u8 pmg_d, u8 pmg_i)
>> +{
>> + u64 default_val;
>> +
>> + default_val = FIELD_PREP(MPAM0_EL1_PARTID_D, partid_d);
>> + default_val |= FIELD_PREP(MPAM0_EL1_PARTID_I, partid_i);
>> + default_val |= FIELD_PREP(MPAM0_EL1_PMG_D, pmg_d);
>> + default_val |= FIELD_PREP(MPAM0_EL1_PMG_I, pmg_i);
>> +
>> + WRITE_ONCE(per_cpu(arm64_mpam_default, cpu), default_val);
>> +}
>> +
>> +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;
>> +
>> + regval = FIELD_PREP(MPAM0_EL1_PARTID_D, partid_d);
>> + regval |= FIELD_PREP(MPAM0_EL1_PARTID_I, partid_i);
>> + regval |= FIELD_PREP(MPAM0_EL1_PMG_D, pmg_d);
>> + regval |= FIELD_PREP(MPAM0_EL1_PMG_I, pmg_i);
>
> Maybe a macro or helper to build regval given this replicates the block in
> mpam_set_cpu_defaults. Obviously ignore if this changes in later patches!

I've added a helper.

>
>> +
>> + WRITE_ONCE(task_thread_info(tsk)->mpam_partid_pmg, regval);
>> +#endif
>> +}
>> +
>> static inline u64 mpam_get_regval(struct task_struct *tsk)
>> {
>> #ifdef CONFIG_ARM64_MPAM
>

Thanks,

Ben