linux-kernel - Re: [PATCH v2 12/45] arm64: mpam: Add helpers to change a task or cpu's MPAM PARTID/PMG values

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Message-ID: <8929c30b-9220-42f9-9a04-db414919a36f@arm.com>
Date: Fri, 9 Jan 2026 09:37:28 +0000
From: Ben Horgan <ben.horgan@....com>
To: "Shaopeng Tan (Fujitsu)" <tan.shaopeng@...itsu.com>
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Subject: Re: [PATCH v2 12/45] arm64: mpam: Add helpers to change a task or
 cpu's MPAM PARTID/PMG values

Hi Shaopeng,

On 1/8/26 10:18, Shaopeng Tan (Fujitsu) wrote:
> Hello  Ben,
> 
>> From: James Morse <james.morse@....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@....com>
>> Signed-off-by: James Morse <james.morse@....com>
>> Signed-off-by: Ben Horgan <ben.horgan@....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 c4cb015e8d8f..a7fd0656bf90 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);
>> +}
>> +
>> +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);
>> +}
>> +
>>  /*
>>   * 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
>> @@ -45,6 +62,17 @@ static inline u64 mpam_get_regval(struct task_struct *tsk)
>>  #endif
>>  }
>>  
>> +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
> 
> Since `mpam_set_task_partid_pmg()` is only called from MPAM, is this necessary?

Yes because mpam_partid_pmg only exists when CONFIG_ARM64_MPAM is enabled.

> 
> Best regards,
> Shaopeng TAN
> 
>> +       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;
>> --
>> 2.43.0

Thanks,

Ben