Re: [PATCH v3 05/12] x86/resctrl: Initialize supported kernel modes for PLZA

[Reinette Chatre]

Hi Babu,

On 4/30/26 4:24 PM, Babu Moger wrote:
> Resctrl subsystem tracks which kernel-mode CLOSID/RMID policies the
> platform can offer via struct resctrl_kmode_cfg and
> resctrl_arch_get_kmode_support(). AMD PLZA (Privilege Level Zero
> Association) is the x86 feature that allows kernel traffic to use an
> assigned CLOSID alone or CLOSID and RMID together.
> 
> Report the available kernel-modes when x86 PLZA is enabled.
> 
> Signed-off-by: Babu Moger <babu.moger@xxxxxxx>
> ---
> v3: New patch to report all the supported kernel mode by arch.
> ---
>  arch/x86/kernel/cpu/resctrl/core.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
> index 4a8717157e3e..699d8bb82875 100644
> --- a/arch/x86/kernel/cpu/resctrl/core.c
> +++ b/arch/x86/kernel/cpu/resctrl/core.c
> @@ -894,6 +894,21 @@ bool resctrl_arch_is_evt_configurable(enum resctrl_event_id evt)
>  	}
>  }
>  
> +/**
> + * resctrl_arch_get_kmode_support() - x86: record which kernel-mode policies hardware supports
> + * @kcfg:	Cumulative snapshot; OR bits into @kcfg->kmode (see &struct resctrl_kmode_cfg).

If this is intended to be a cumulative snapshot this is a very subtle requirement
for architectures to "do the right thing" here. To make this more robust I think it will be
simpler if resctrl fs boots with resctrl_kcfg initialized to expected defaults. 
Instead of this callback resctrl can add resctrl_set_kmode_support(u32 kmodes)
that the architecture *may* use to further initialize the kmodes supported by it. This
function is implemented by resctrl fs, instead of architecture, and it can fail if
architecture does not support INHERIT_CTRL_AND_MON. This will help to keep
struct resctrl_kmode_cfg private to resctrl fs while enforcing any assumptions about
which modes are required to be supported.

> + *
> + * When PLZA is present (CPUID X86_FEATURE_PLZA), the kernel may assign a CLOSID
> + * for kernel work alone or assign CLOSID and RMID together.  Advertise both
> + * assign-style modes in @kcfg->kmode using &enum resctrl_kernel_modes indices.
> + */
> +void resctrl_arch_get_kmode_support(struct resctrl_kmode_cfg *kcfg)
> +{
> +	if (rdt_cpu_has(X86_FEATURE_PLZA))
> +		kcfg->kmode |= BIT(GLOBAL_ASSIGN_CTRL_INHERIT_MON_PER_CPU) |
> +				BIT(GLOBAL_ASSIGN_CTRL_ASSIGN_MON_PER_CPU);
> +}
> +
>  static __init bool get_mem_config(void)
>  {
>  	struct rdt_hw_resource *hw_res = &rdt_resources_all[RDT_RESOURCE_MBA];

Reinette