linux-kernel - Re: [PATCH v3 09/47] arm64: mpam: Add cpu

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Message-ID: <5c79d642-cd95-4c6b-9eeb-bd055719ca91@redhat.com>
Date: Mon, 19 Jan 2026 14:50:16 +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,
 will@...nel.org, corbet@....net, maz@...nel.org, oupton@...nel.org,
 joey.gouly@....com, suzuki.poulose@....com, kvmarm@...ts.linux.dev
Subject: Re: [PATCH v3 09/47] arm64: mpam: Add cpu_pm notifier to restore MPAM
 sysregs

Hi Ben,

On 1/13/26 12:58 AM, Ben Horgan wrote:
> From: James Morse <james.morse@....com>
> 
> The MPAM system registers will be lost if the CPU is reset during PSCI's
> CPU_SUSPEND.
> 
> Add a PM notifier to restore them.
> 
> mpam_thread_switch(current) can't be used as this won't make any changes if
> the in-memory copy says the register already has the correct value. In
> reality the system register is UNKNOWN out of reset.
> 
> Reviewed-by: Jonathan Cameron <jonathan.cameron@...wei.com>
> Signed-off-by: James Morse <james.morse@....com>
> Signed-off-by: Ben Horgan <ben.horgan@....com>
> ---
>   arch/arm64/kernel/mpam.c | 30 ++++++++++++++++++++++++++++++
>   1 file changed, 30 insertions(+)
> 

One question below...

> diff --git a/arch/arm64/kernel/mpam.c b/arch/arm64/kernel/mpam.c
> index e6feff2324ac..dbe0a2d05abb 100644
> --- a/arch/arm64/kernel/mpam.c
> +++ b/arch/arm64/kernel/mpam.c
> @@ -4,6 +4,7 @@
>   #include <asm/mpam.h>
>   
>   #include <linux/arm_mpam.h>
> +#include <linux/cpu_pm.h>
>   #include <linux/jump_label.h>
>   #include <linux/percpu.h>
>   
> @@ -13,12 +14,41 @@ DEFINE_PER_CPU(u64, arm64_mpam_current);
>   
>   u64 arm64_mpam_global_default;
>   
> +static int mpam_pm_notifier(struct notifier_block *self,
> +			    unsigned long cmd, void *v)
> +{
> +	u64 regval;
> +	int cpu = smp_processor_id();
> +
> +	switch (cmd) {
> +	case CPU_PM_EXIT:
> +		/*
> +		 * Don't use mpam_thread_switch() as the system register
> +		 * value has changed under our feet.
> +		 */
> +		regval = READ_ONCE(per_cpu(arm64_mpam_current, cpu));
> +		write_sysreg_s(regval, SYS_MPAM1_EL1);
> +		isb();
> +
> +		write_sysreg_s(regval, SYS_MPAM0_EL1);
> +
> +		return NOTIFY_OK;
> +	default:
> +		return NOTIFY_DONE;
> +	}
> +}
> +
> +static struct notifier_block mpam_pm_nb = {
> +	.notifier_call = mpam_pm_notifier,
> +};
> +
>   static int __init arm64_mpam_register_cpus(void)
>   {
>   	u64 mpamidr = read_sanitised_ftr_reg(SYS_MPAMIDR_EL1);
>   	u16 partid_max = FIELD_GET(MPAMIDR_EL1_PARTID_MAX, mpamidr);
>   	u8 pmg_max = FIELD_GET(MPAMIDR_EL1_PMG_MAX, mpamidr);
>   
> +	cpu_pm_register_notifier(&mpam_pm_nb);

Need we ensure MPAM capability exists in the hardware before the notifier is registerred?
Otherwise, mpam_pm_notifier() can accesses SYS_MPAM0_EL1 and SYS_MPAM1_EL1 system registers
which may not supported by the hardware.

>   	return mpam_register_requestor(partid_max, pmg_max);
>   }
>   /* Must occur before mpam_msc_driver_init() from subsys_initcall() */

Thanks,
Gavin