Re: [PATCH v3 04/12] x86,fs/resctrl: Program PLZA through kmode arch hooks

[Reinette Chatre]

Hi Babu,

On 4/30/26 4:24 PM, Babu Moger wrote:
> AMD Privilege Level Zero Association (PLZA) exposes kernel CLOSID/RMID
> association through MSR_IA32_PQR_PLZA_ASSOC.  Generic resctrl already
> tracks supported and effective kernel-mode policy in struct
> resctrl_kmode_cfg, but the architecture layer needs a callable entry point
> that can push those values into per-CPU hardware on a chosen CPU mask.
> 
> Declare resctrl_arch_configure_kmode() in linux/resctrl.h with kernel-doc.
> Implement it on x86: add an SMP callback that writes
> MSR_IA32_PQR_PLZA_ASSOC on each targeted CPU, and use on_each_cpu_mask()
> for the broadcast.

Above is clear from the patch. Please start with focus on why this patch is
needed. 

> 
> The hook is unused in this patch; later patches in the series wire it into

Similar to previous work: write changelog in imperative tone and do not
refer to patches in series but instead let each patch stand on its own.

> generic resctrl when an effective kernel-mode policy is selected or a CPU
> mask changes.
> 
> Signed-off-by: Babu Moger <babu.moger@xxxxxxx>
> ---


> ---
>  arch/x86/kernel/cpu/resctrl/ctrlmondata.c | 35 +++++++++++++++++++++++
>  include/linux/resctrl.h                   | 10 +++++++
>  2 files changed, 45 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> index b20e705606b8..68f1cf503904 100644
> --- a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> +++ b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> @@ -131,3 +131,38 @@ int resctrl_arch_io_alloc_enable(struct rdt_resource *r, bool enable)
>  
>  	return 0;
>  }
> +
> +/*
> + * SMP call-function callback: each CPU writes its own MSR_IA32_PQR_PLZA_ASSOC
> + * (AMD PLZA).  Invoked via on_each_cpu_mask() with wait=1 so the on-stack
> + * union pointed at by @arg is safe.
> + */
> +static void resctrl_kmode_set_one_amd(void *arg)
> +{
> +	union msr_pqr_plza_assoc *plza = arg;
> +
> +	wrmsrl(MSR_IA32_PQR_PLZA_ASSOC, plza->full);

fyi ...
commit 2232959db26d ("x86/msr: Switch wrmsrl() users to wrmsrq()") 
commit b5884070f9da ("x86/msr: Remove wrmsrl()") 

> +}
> +
> +/**
> + * resctrl_arch_configure_kmode() - x86/AMD: program PLZA MSR on a CPU subset
> + * @cpu_mask:	CPUs to receive the update (see on_each_cpu_mask() for online subset).

Why is the caveat added? Will resctrl ever provide offline CPUs in the mask?

> + * @closid:	CLOSID field written into the MSR with CLOSID_EN set.
> + * @rmid:	RMID field written into the MSR with RMID_EN set.
> + * @enable:	Value for the PLZA_EN split field.

Please describe the meaning of the fields instead the mechanics of the code
that are obvious.

> + *
> + * Context: Do not call with IRQs off or from IRQ context except as allowed for
> + * on_each_cpu_mask(); see kernel/smp.c.

Why is this context caveat needed? 

> + */
> +void resctrl_arch_configure_kmode(cpumask_var_t cpu_mask, u32 closid, u32 rmid, bool enable)

Please replace "cpumask_var_t cpu_mask" with "const struct cpumask *cpu_mask".

> +{
> +	union msr_pqr_plza_assoc plza = { 0 };
> +
> +	plza.split.rmid = rmid;
> +	plza.split.rmid_en = 1;
> +	plza.split.closid = closid;
> +	plza.split.closid_en = 1;
> +	plza.split.plza_en = enable;
> +
> +	on_each_cpu_mask(cpu_mask, resctrl_kmode_set_one_amd, &plza, 1);
> +}

function self has been discussed already

> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
> index ce28418df00f..570918e57e24 100644
> --- a/include/linux/resctrl.h
> +++ b/include/linux/resctrl.h
> @@ -712,6 +712,16 @@ bool resctrl_arch_get_io_alloc_enabled(struct rdt_resource *r);
>   */
>  void resctrl_arch_get_kmode_support(struct resctrl_kmode_cfg *kcfg);
>  
> +/**
> + * resctrl_arch_configure_kmode() - Program MSR_IA32_PQR_PLZA_ASSOC on CPUs in @cpu_mask
> + * @cpu_mask:	Target CPUs; on_each_cpu_mask() applies the callback on the online subset.
> + * @closid:	CLOSID written to the MSR with CLOSID_EN set.
> + * @rmid:	RMID written to the MSR with RMID_EN set.
> + * @enable:	PLZA_EN field value for this update.

This is a resctrl fs API - please replace all the AMD architecture specific implementation details
with what the parameters actually mean/represent.

> + */
> +void resctrl_arch_configure_kmode(cpumask_var_t cpu_mask, u32 closid, u32 rmid,
> +				  bool enable);
> +
>  extern unsigned int resctrl_rmid_realloc_threshold;
>  extern unsigned int resctrl_rmid_realloc_limit;
>  

Reinette