linux-kernel - Re: [RFC PATCH 04/38] arm64: mpam: Add cpu

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Message-ID: <a9e70c54-318a-43d3-b5d8-2d102390c363@arm.com>
Date: Thu, 11 Dec 2025 13:41:06 +0000
From: Ben Horgan <ben.horgan@....com>
To: James Morse <james.morse@....com>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org
Cc: 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,
 Jonathan Cameron <jonathan.cameron@...wei.com>, Gavin Shan
 <gshan@...hat.com>, rohit.mathew@....com, reinette.chatre@...el.com,
 Punit Agrawal <punit.agrawal@....qualcomm.com>
Subject: Re: [RFC PATCH 04/38] arm64: mpam: Add cpu_pm notifier to restore
 MPAM sysregs

Hi James,

On 12/5/25 21:58, James Morse wrote:
> 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.

If CONFIG_ARM64_MPAM is not enabled then the PM notifier is never
registered but the MPAMx_ELy will still come out of reset with UNKNOWN
values and we need to set the PARTID values back to 0 as the MSC are
only guaranteed to provide good defaults for that value. (We aren't
using the MPAM driver to initialise the other PARTID.)

> 
> Signed-off-by: James Morse <james.morse@....com>
> ---
>  arch/arm64/kernel/mpam.c | 30 ++++++++++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
> 
> 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);
>  	return mpam_register_requestor(partid_max, pmg_max);
>  }
>  /* Must occur before mpam_msc_driver_init() from subsys_initcall() */

Thanks,

Ben