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

Ben Horgan
2025-12-11T11:23:35Z

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@arm.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