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

James Morse
2025-12-05T21:59:33Z

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.

MPAM has two CPU system registers that is used to hold the PARTID and PMG
values that traffic generated at each exception level will use. These can be
set per-task by the resctrl file system. (resctrl is the defacto interface
for controlling this stuff).

Add a helper to switch this.

struct task_struct's separate CLOSID and RMID fields are insufficient
to implement resctrl using MPAM, as resctrl can change the PARTID (CLOSID)
and PMG (sort of like the RMID) separately. On x86, the rmid is an
independent number, so a race that writes a mismatched closid and rmid
into hardware is benign. On arm64, the pmg bits extend the partid.
(i.e. partid-5 has a pmg-0 that is not the same as partid-6's pmg-0).
In this case, mismatching the values will 'dirty' a pmg value that
resctrl believes is clean, and is not tracking with its 'limbo' code.

To avoid this, the partid and pmg are always read and written as a pair.
Instead of making struct task_struct's closid and rmid fields an
endian-unsafe union, add the value to struct thread_info and always use
READ_ONCE()/WRITE_ONCE() when accessing this field.

Resctrl allows a per-cpu 'default' value to be set, this overrides the
values when scheduling a task in the default control-group, which has
PARTID 0. The way 'code data prioritisation' gets emulated means the
register value for the default group needs to be a variable.

The current system register value is kept in a per-cpu variable to
avoid writing to the system register if the value isn't going to change.
Writes to this register may reset the hardware state for regulating
bandwidth.

Finally, there is no reason to context switch these registers unless
there is a driver changing the values in struct task_struct. Hide
the whole thing behind a static key. This also allows the driver to
disable MPAM in response to errors reported by hardware. Move the
existing static key to belong to the arch code, as in the future
the MPAM driver may become a loadable module.

All this should depend on whether there is an MPAM driver, hide
it behind CONFIG_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.
+ *
+ * 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;
#endif
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;
--
2.39.5