linux-kernel - Re: [RFC PATCH 02/38] arm64: mpam: Re-initialise MPAM regs when CPU comes online

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Message-ID: <b922ff73-c583-482e-8925-ad7758280705@arm.com>
Date: Thu, 11 Dec 2025 11:23:29 +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 02/38] arm64: mpam: Re-initialise MPAM regs when CPU
 comes online

Hi James,

On 12/9/25 15:13, Ben Horgan wrote:
> Hi James,
> 
> On 12/5/25 21:58, James Morse wrote:
>> Now that the MPAM system registers are expected to have values that change,
>> reprogram them based on struct task_struct when a CPU is brought online.
>>
>> Previously MPAM's 'default PARTID' of 0 was used this is the PARTID that
>> hardware guarantees to reset. Because there are a limited number of
>> PARTID, this value is exposed to user space, meaning resctrl changes
>> to the resctrl default group would also affect kernel threads.
>> Instead, use the task's PARTID value for kernel work on behalf of
>> user-space too.
>>
>> Signed-off-by: James Morse <james.morse@....com>
>> ---
>>  arch/arm64/kernel/cpufeature.c | 18 +++++++++++-------
>>  1 file changed, 11 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>> index 5ed401ff79e3..429128a181ac 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c
>> @@ -86,6 +86,7 @@
>>  #include <asm/kvm_host.h>
>>  #include <asm/mmu.h>
>>  #include <asm/mmu_context.h>
>> +#include <asm/mpam.h>
>>  #include <asm/mte.h>
>>  #include <asm/hypervisor.h>
>>  #include <asm/processor.h>
>> @@ -2439,13 +2440,16 @@ test_has_mpam(const struct arm64_cpu_capabilities *entry, int scope)
>>  static void
>>  cpu_enable_mpam(const struct arm64_cpu_capabilities *entry)
>>  {
>> -	/*
>> -	 * Access by the kernel (at EL1) should use the reserved PARTID
>> -	 * which is configured unrestricted. This avoids priority-inversion
>> -	 * where latency sensitive tasks have to wait for a task that has
>> -	 * been throttled to release the lock.
>> -	 */
>> -	write_sysreg_s(0, SYS_MPAM1_EL1);
>> +	int cpu = smp_processor_id();
>> +	u64 regval = 0;
>> +
>> +	if (IS_ENABLED(CONFIG_MPAM))
>> +		regval = READ_ONCE(per_cpu(arm64_mpam_current, cpu));
> 
> CONFIG_MPAM -> CONFIG_ARM64_MPAM

Actually, this code is only run before the mpam enablement is finished,
importantly before the mpam_enabled static key is set, and so
arm64_mpam_current is still 0 for every cpu. For cpus that are brought
up after boot time this is never run.

As SYS_MPAM0_EL1 and SYS_MPAM1_EL1 are unknown out of reset we should
set them to 0 whenever a cpu comes online to make sure they initially
use PARTID 0 as that is the only one guaranteed to have sensible
defaults in the MSC. Once the mpam driver has configured the MSC we can
start setting other values.

> 
>> +
>> +	write_sysreg_s(regval, SYS_MPAM1_EL1);
>> +	isb();
>> +
>> +	write_sysreg_s(regval, SYS_MPAM0_EL1);
>>  }
>>  
>>  static bool
> 
> Thanks,
> 
> Ben
> 
> 

Thanks,

Ben