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

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Message-ID: <1389bd4b-494c-42da-84bf-89d0055e2d16@arm.com>
Date: Mon, 19 Jan 2026 15:08:39 +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 09/47] arm64: mpam: Add cpu_pm notifier to restore MPAM
 sysregs

Hi Gavin,

On 1/19/26 06:50, Gavin Shan wrote:
> 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.

Ah yes, I'll add a system_supports_mpam() check here.

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

Thanks,

Ben