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

[Ben Horgan <ben.horgan@arm.com>]

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

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

Thanks,

Ben