Re: [RFC PATCH 01/38] arm64: mpam: Context switch the MPAM registers

Ben Horgan
2025-12-09T14:49:06Z

Hi James,

On 12/5/25 21:58, James Morse wrote:
> MPAM allows traffic in the SoC to be labeled by the OS, these labels
> are used to apply policy in caches and bandwidth regulators, and to
> monitor traffic in the SoC. The label is made up of a PARTID and PMG
> value. The x86 equivalent calls these CLOSID and RMID, but they don't
> map precisely.
>
[...]
>
> All this should depend on whether there is an MPAM driver, hide
> it behind CONFIG_MPAM.

CONFIG_MPAM -> CONFIG_ARM64_MPAM

>
> CC: Amit Singh Tomar <amitsinght@marvell.com>
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
> arch/arm64/Kconfig | 2 +
> arch/arm64/include/asm/mpam.h | 74 ++++++++++++++++++++++++++++
> arch/arm64/include/asm/thread_info.h | 3 ++
> arch/arm64/kernel/Makefile | 1 +
> arch/arm64/kernel/mpam.c | 13 +++++
> arch/arm64/kernel/process.c | 7 +++
> drivers/resctrl/mpam_devices.c | 2 -
> drivers/resctrl/mpam_internal.h | 2 +
> 8 files changed, 102 insertions(+), 2 deletions(-)
> create mode 100644 arch/arm64/include/asm/mpam.h
> create mode 100644 arch/arm64/kernel/mpam.c
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 004d58cfbff8..558baa9e7c08 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -2048,6 +2048,8 @@ config ARM64_MPAM
>
> MPAM is exposed to user-space via the resctrl pseudo filesystem.
>
> + This option enables the extra context switch code.
> +
> endmenu # "ARMv8.4 architectural features"
>
> menu "ARMv8.5 architectural features"
> diff --git a/arch/arm64/include/asm/mpam.h b/arch/arm64/include/asm/mpam.h
> new file mode 100644
> index 000000000000..86a55176f884
> --- /dev/null
> +++ b/arch/arm64/include/asm/mpam.h
> @@ -0,0 +1,74 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/* Copyright (C) 2025 Arm Ltd. */
> +
> +#ifndef __ASM__MPAM_H
> +#define __ASM__MPAM_H
> +
> +#include <linux/bitops.h>
> +#include <linux/init.h>
> +#include <linux/jump_label.h>
> +#include <linux/percpu.h>
> +#include <linux/sched.h>
> +
> +#include <asm/cpucaps.h>
> +#include <asm/cpufeature.h>
> +#include <asm/sysreg.h>
> +
> +DECLARE_STATIC_KEY_FALSE(mpam_enabled);
> +DECLARE_PER_CPU(u64, arm64_mpam_default);
> +DECLARE_PER_CPU(u64, arm64_mpam_current);
> +
> +/*
> + * The value of the MPAM0_EL1 sysreg when a task is in resctrl's default group.
> + * This is used by the context switch code to use the resctrl CPU property
> + * instead. The value is modified when CDP is enabled/disabled by mounting
> + * the resctrl filesystem.
> + */
> +extern u64 arm64_mpam_global_default;
> +
> +/*
> + * The resctrl filesystem writes to the partid/pmg values for threads and CPUs,
> + * which may race with reads in __mpam_sched_in(). Ensure only one of the old
> + * or new values are used. Particular care should be taken with the pmg field
> + * as __mpam_sched_in() may read a partid and pmg that don't match, causing
> + * this value to be stored with cache allocations, despite being considered
> + * 'free' by resctrl.

__mpam_sched_in() -> mpam_thread_switch()
This is called from resctrl_arch_sched_in().

> + *
> + * A value in struct thread_info is used instead of struct task_struct as the
> + * cpu's u64 register format is used, but struct task_struct has two u32'.
> + */
> +static inline u64 mpam_get_regval(struct task_struct *tsk)
> +{
> +#ifdef CONFIG_ARM64_MPAM
> + return READ_ONCE(task_thread_info(tsk)->mpam_partid_pmg);
> +#else
> + return 0;
> +#endif
> +}
> +
> +static inline void mpam_thread_switch(struct task_struct *tsk)
> +{
> + u64 oldregval;
> + int cpu = smp_processor_id();
> + u64 regval = mpam_get_regval(tsk);
> +
> + if (!IS_ENABLED(CONFIG_ARM64_MPAM) ||
> + !static_branch_likely(&mpam_enabled))
> + return;
> +
> + if (regval == READ_ONCE(arm64_mpam_global_default))
> + regval = READ_ONCE(per_cpu(arm64_mpam_default, cpu));
> +
> + oldregval = READ_ONCE(per_cpu(arm64_mpam_current, cpu));
> + if (oldregval == regval)
> + return;
> +
> + write_sysreg_s(regval, SYS_MPAM1_EL1);
> + isb();
> +
> + /* Synchronising the EL0 write is left until the ERET to EL0 */
> + write_sysreg_s(regval, SYS_MPAM0_EL1);
> +
> + WRITE_ONCE(per_cpu(arm64_mpam_current, cpu), regval);
> +}
> +#endif /* __ASM__MPAM_H */
> diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
> index f241b8601ebd..c226dabd5019 100644
> --- a/arch/arm64/include/asm/thread_info.h
> +++ b/arch/arm64/include/asm/thread_info.h
> @@ -41,6 +41,9 @@ struct thread_info {
> #ifdef CONFIG_SHADOW_CALL_STACK
> void *scs_base;
> void *scs_sp;
> +#endif
> +#ifdef CONFIG_ARM64_MPAM
> + u64 mpam_partid_pmg;
> #end if
> u32 cpu;
> };
> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
> index 76f32e424065..15979f366519 100644
> --- a/arch/arm64/kernel/Makefile
> +++ b/arch/arm64/kernel/Makefile
> @@ -67,6 +67,7 @@ obj-$(CONFIG_CRASH_DUMP) += crash_dump.o
> obj-$(CONFIG_VMCORE_INFO) += vmcore_info.o
> obj-$(CONFIG_ARM_SDE_INTERFACE) += sdei.o
> obj-$(CONFIG_ARM64_PTR_AUTH) += pointer_auth.o
> +obj-$(CONFIG_ARM64_MPAM) += mpam.o
> obj-$(CONFIG_ARM64_MTE) += mte.o
> obj-y += vdso-wrap.o
> obj-$(CONFIG_COMPAT_VDSO) += vdso32-wrap.o
> diff --git a/arch/arm64/kernel/mpam.c b/arch/arm64/kernel/mpam.c
> new file mode 100644
> index 000000000000..9866d2ca0faa
> --- /dev/null
> +++ b/arch/arm64/kernel/mpam.c
> @@ -0,0 +1,13 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (C) 2025 Arm Ltd. */
> +
> +#include <asm/mpam.h>
> +
> +#include <linux/jump_label.h>
> +#include <linux/percpu.h>
> +
> +DEFINE_STATIC_KEY_FALSE(mpam_enabled);
> +DEFINE_PER_CPU(u64, arm64_mpam_default);
> +DEFINE_PER_CPU(u64, arm64_mpam_current);
> +
> +u64 arm64_mpam_global_default;
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index fba7ca102a8c..b510c0699313 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -51,6 +51,7 @@
> #include <asm/fpsimd.h>
> #include <asm/gcs.h>
> #include <asm/mmu_context.h>
> +#include <asm/mpam.h>
> #include <asm/mte.h>
> #include <asm/processor.h>
> #include <asm/pointer_auth.h>
> @@ -737,6 +738,12 @@ struct task_struct *__switch_to(struct task_struct *prev,
> if (prev->thread.sctlr_user != next->thread.sctlr_user)
> update_sctlr_el1(next->thread.sctlr_user);
>
> + /*
> + * MPAM thread switch happens after the DSB to ensure prev's accesses
> + * use prev's MPAM settings.
> + */
> + mpam_thread_switch(next);
> +
> /* the actual thread switch */
> last = cpu_switch_to(prev, next);
>
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index 0b5b158e1aaf..2996ad93fc3e 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -29,8 +29,6 @@
>
> #include "mpam_internal.h"
>
> -DEFINE_STATIC_KEY_FALSE(mpam_enabled); /* This moves to arch code */
> -
> /*
> * mpam_list_lock protects the SRCU lists when writing. Once the
> * mpam_enabled key is enabled these lists are read-only,
> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
> index e79c3c47259c..4508a6654fe0 100644
> --- a/drivers/resctrl/mpam_internal.h
> +++ b/drivers/resctrl/mpam_internal.h
> @@ -17,6 +17,8 @@
> #include <linux/srcu.h>
> #include <linux/types.h>
>
> +#include <asm/mpam.h>
> +
> #define MPAM_MSC_MAX_NUM_RIS 16
>
> struct platform_device;

The DECLARE_STATIC_KEY_FALSE(mpam_enabled) on the next line can now be
removed.

Thanks,

Ben