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: <57a015c3-9a4b-478b-8cb6-6c61f84c0b74@redhat.com>
Date: Mon, 19 Jan 2026 15:01:41 +0800
From: Gavin Shan <gshan@...hat.com>
To: Ben Horgan <ben.horgan@....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 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(...);

> +
> +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