Re: [PATCH v3 06/47] arm64: mpam: Context switch the MPAM registers

Ben Horgan
2026-01-19T14:00:14Z

Hi Gavin, Jonathan,

On 1/15/26 12:09, Jonathan Cameron wrote:
> On Thu, 15 Jan 2026 14:47:28 +0800
> Gavin Shan <gshan@redhat.com> wrote:
>
>> Hi Ben,
>>
>> On 1/13/26 12:58 AM, Ben Horgan wrote:
>>> From: James Morse <james.morse@arm.com>
>>>
>>> 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. This requires a new u64 field. In struct task_struct there are two
>>> u32, rmid and closid for the x86 case, but as we can't use them here do
>>> something else. Add this new field, mpam_partid_pmg, to struct thread_info
>>> to avoid adding more architecture specific code to struct task_struct.
>>> 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_ARM64_MPAM.
>>>
>>> CC: Amit Singh Tomar <amitsinght@marvell.com>
>>> Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
>>> Signed-off-by: James Morse <james.morse@arm.com>
>>> Signed-off-by: Ben Horgan <ben.horgan@arm.com>
>>> ---
>>> Changes since rfc:
>>> CONFIG_MPAM -> CONFIG_ARM64_MPAM in commit message
>>> Remove extra DECLARE_STATIC_KEY_FALSE
>>> Function name in comment, __mpam_sched_in() -> mpam_thread_switch()
>>> Remove unused headers
>>> Expand comment (Jonathan)
>>>
>>> Changes since v2:
>>> Tidy up ifdefs
>>> ---
>>> arch/arm64/Kconfig | 2 +
>>> arch/arm64/include/asm/mpam.h | 67 ++++++++++++++++++++++++++++
>>> 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 | 4 +-
>>> 8 files changed, 95 insertions(+), 4 deletions(-)
>>> create mode 100644 arch/arm64/include/asm/mpam.h
>>> create mode 100644 arch/arm64/kernel/mpam.c
>>>
>>
>> With the following nitpick addressed:
>>
>
> I commented on the nitpick.
>
>> Reviewed-by: Gavin Shan <gshan@redhat.com>
>
>>> 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>
>>> +
>>
>> Nitpick: Needn't include those two header files since they have been included
>> to <asm/mpam.h>
>
> That is a non obvious include chain that we should not rely on.
> Please keep the headers and continue to follow include what you use
> style (with exceptions when a given header is clearly documented as always including
> another like some of the bit map stuff.) It is more obviously correct and
> causes less grief if headers get refactored in future.

Keeping the includes here makes sense to me too. Gavin, are you ok with
keeping this as is?

>
>>
>>> +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;
>
>>
>

Thanks,

Ben