[PATCH v2 12/45] arm64: mpam: Add helpers to change a task or cpu's MPAM PARTID/PMG values

[Shaopeng Tan (Fujitsu)]

Hello  Ben,

> From: James Morse <james.morse at arm.com>
> 
> Care must be taken when modifying the PARTID and PMG of a task in any
> per-task structure as writing these values may race with the task being
> scheduled in, and reading the modified values.
> 
> Add helpers to set the task properties, and the CPU default value.  These
> use WRITE_ONCE() that pairs with the READ_ONCE() in mpam_get_regval() to
> avoid causing torn values.
> 
> CC: Dave Martin <Dave.Martin at arm.com>
> Signed-off-by: James Morse <james.morse at arm.com>
> Signed-off-by: Ben Horgan <ben.horgan at arm.com>
> ---
> Changes since rfc:
> Keep comment attached to mpam_get_regval()
> Add internal helper, __mpam_regval() (Jonathan)
> ---
>  arch/arm64/include/asm/mpam.h | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/mpam.h b/arch/arm64/include/asm/mpam.h
> index c4cb015e8d8f..a7fd0656bf90 100644
> --- a/arch/arm64/include/asm/mpam.h
> +++ b/arch/arm64/include/asm/mpam.h
> @@ -4,6 +4,7 @@
>  #ifndef __ASM__MPAM_H
>  #define __ASM__MPAM_H
>  
> +#include <linux/bitfield.h>
>  #include <linux/jump_label.h>
>  #include <linux/percpu.h>
>  #include <linux/sched.h>
> @@ -22,6 +23,22 @@ DECLARE_PER_CPU(u64, arm64_mpam_current);
>   */
>  extern u64 arm64_mpam_global_default;
>  
> +static inline u64 __mpam_regval(u16 partid_d, u16 partid_i, u8 pmg_d, u8 pmg_i)
> +{
> +       return FIELD_PREP(MPAM0_EL1_PARTID_D, partid_d) |
> +               FIELD_PREP(MPAM0_EL1_PARTID_I, partid_i) |
> +               FIELD_PREP(MPAM0_EL1_PMG_D, pmg_d) |
> +               FIELD_PREP(MPAM0_EL1_PMG_I, pmg_i);
> +}
> +
> +static inline void mpam_set_cpu_defaults(int cpu, u16 partid_d, u16 partid_i,
> +                                        u8 pmg_d, u8 pmg_i)
> +{
> +       u64 default_val = __mpam_regval(partid_d, partid_i, pmg_d, pmg_i);
> +
> +       WRITE_ONCE(per_cpu(arm64_mpam_default, cpu), default_val);
> +}
> +
>  /*
>   * 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
> @@ -45,6 +62,17 @@ static inline u64 mpam_get_regval(struct task_struct *tsk)
>  #endif
>  }
>  
> +static inline void mpam_set_task_partid_pmg(struct task_struct *tsk,
> +                                           u16 partid_d, u16 partid_i,
> +                                           u8 pmg_d, u8 pmg_i)
> +{
> +#ifdef CONFIG_ARM64_MPAM

Since `mpam_set_task_partid_pmg()` is only called from MPAM, is this necessary?

Best regards,
Shaopeng TAN

> +       u64 regval = __mpam_regval(partid_d, partid_i, pmg_d, pmg_i);
> +
> +       WRITE_ONCE(task_thread_info(tsk)->mpam_partid_pmg, regval);
> +#endif
> +}
> +
>  static inline void mpam_thread_switch(struct task_struct *tsk)
>  {
>          u64 oldregval;
> --
> 2.43.0