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

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Message-ID: <a48e749d-fdeb-4306-9324-0f62fc72b589@arm.com>
Date: Mon, 19 Jan 2026 15:49:05 +0000
From: Ben Horgan <ben.horgan@....com>
To: Gavin Shan <gshan@...hat.com>
Cc: amitsinght@...vell.com, baisheng.gao@...soc.com,
 baolin.wang@...ux.alibaba.com, carl@...amperecomputing.com,
 dave.martin@....com, david@...nel.org, dfustini@...libre.com,
 fenghuay@...dia.com, james.morse@....com, jonathan.cameron@...wei.com,
 kobak@...dia.com, lcherian@...vell.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 peternewman@...gle.com, punit.agrawal@....qualcomm.com,
 quic_jiles@...cinc.com, reinette.chatre@...el.com, rohit.mathew@....com,
 scott@...amperecomputing.com, sdonthineni@...dia.com,
 tan.shaopeng@...itsu.com, xhao@...ux.alibaba.com, catalin.marinas@....com,
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 joey.gouly@....com, suzuki.poulose@....com, kvmarm@...ts.linux.dev
Subject: Re: [PATCH v3 11/47] arm64: mpam: Add helpers to change a task or
 cpu's MPAM PARTID/PMG values

Hi Gavin,

On 1/19/26 07:01, Gavin Shan wrote:
> Hi Ben,
> 
> On 1/13/26 12:58 AM, Ben Horgan wrote:
>> 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>
>> Reviewed-by: Jonathan Cameron <jonathan.cameron@...wei.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 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(...);

Sure, I'll align these.

> 
>> +
>> +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'.

I'll move the #ifdef CONFIG_ARM64_MPAM up to cover these.

> 
>>   /*
>>    * 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
> 

Thanks,

Ben