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

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Message-ID: <e959f679-9c3c-41d9-b607-ba2e59ed37ee@arm.com>
Date: Fri, 19 Dec 2025 11:56:17 +0000
From: Ben Horgan <ben.horgan@....com>
To: Jonathan Cameron <jonathan.cameron@...wei.com>,
 James Morse <james.morse@....com>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 D Scott Phillips OS <scott@...amperecomputing.com>,
 carl@...amperecomputing.com, lcherian@...vell.com,
 bobo.shaobowang@...wei.com, tan.shaopeng@...itsu.com,
 baolin.wang@...ux.alibaba.com, Jamie Iles <quic_jiles@...cinc.com>,
 Xin Hao <xhao@...ux.alibaba.com>, peternewman@...gle.com,
 dfustini@...libre.com, amitsinght@...vell.com,
 David Hildenbrand <david@...nel.org>, Dave Martin <dave.martin@....com>,
 Koba Ko <kobak@...dia.com>, Shanker Donthineni <sdonthineni@...dia.com>,
 fenghuay@...dia.com, baisheng.gao@...soc.com, Gavin Shan <gshan@...hat.com>,
 rohit.mathew@....com, reinette.chatre@...el.com,
 Punit Agrawal <punit.agrawal@....qualcomm.com>
Subject: Re: [RFC PATCH 05/38] arm64: mpam: Add helpers to change a task or
 cpu's MPAM PARTID/PMG values

Hi Jonathan,

On 12/18/25 10:44, Jonathan Cameron wrote:
> On Fri, 5 Dec 2025 21:58:28 +0000
> James Morse <james.morse@....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@....com>
>> CC: Ben Horgan <ben.horgan@....com>
>> Signed-off-by: James Morse <james.morse@....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