Message-ID: <c38d48e4-a4c4-4d35-8ca7-ffef83e03077@arm.com>
Date: Fri, 9 Jan 2026 09:28:17 +0000
From: Ben Horgan <ben.horgan@....com>
To: "Shaopeng Tan (Fujitsu)" <tan.shaopeng@...itsu.com>
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Subject: Re: [PATCH v2 07/45] arm64: mpam: Context switch the MPAM registers

Hi Shaopeng,

On 1/8/26 10:06, Shaopeng Tan (Fujitsu) wrote:
> Hello Ben,
> 
>> From: James Morse <james.morse@....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.
>> 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_ARM64_MPAM.
>>
>> CC: Amit Singh Tomar <amitsinght@...vell.com>
>> Signed-off-by: James Morse <james.morse@....com>
>> Signed-off-by: Ben Horgan <ben.horgan@....com>
>> ---
>> 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)
>> ---
>>  arch/arm64/Kconfig                   |  2 +
>>  arch/arm64/include/asm/mpam.h        | 73 ++++++++++++++++++++++++++++
>>  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, 101 insertions(+), 4 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 93173f0a09c7..cdcc5b76a110 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -2049,6 +2049,8 @@ 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..2ab3dca6977c
>> --- /dev/null
>> +++ b/arch/arm64/include/asm/mpam.h
>> @@ -0,0 +1,73 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +/* Copyright (C) 2025 Arm Ltd. */
>> +
>> +#ifndef __ASM__MPAM_H
>> +#define __ASM__MPAM_H
>> +
>> +#include <linux/jump_label.h>
>> +#include <linux/percpu.h>
>> +#include <linux/sched.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_thread_switch(). Ensure only one of the old
>> + * or new values are used. Particular care should be taken with the pmg field as
>> + * mpam_thread_switch() 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. 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. Creating a union would mean only accesses from the created u64 would be
>> + * endian safe and so be less clear.
>> + */
>> +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);
>> +}
> 
> How about changing the code as follows? (Refer to "mte_thread_switch(next);" in "arch/arm64/kernel/process.c")
> 
>  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
>  }
> 
> +#ifdef CONFIG_ARM64_MPAM
>  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))
> +       if (!static_branch_likely(&mpam_enabled))
>                 return;
> 
>         if (regval == READ_ONCE(arm64_mpam_global_default))
> @@ -101,4 +96,8 @@ static inline void mpam_thread_switch(struct task_struct *tsk)
> 
>         WRITE_ONCE(per_cpu(arm64_mpam_current, cpu), regval);
>  }
> +#else
> +static inline void mpam_thread_switch(struct task_struct *tsk){}
> +#endif

Yes, this makes the ifdefs a bit clearer. I'll update.

> +
> Best regards,
> Shaopeng TAN
> 
Thanks,

Ben

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