Re: [RESEND PATCH v4 07/15] x86/resctrl: Expose the supported PLZA kernel-mode policies during init

[Babu Moger]

On 7/7/26 16:50, Babu Moger wrote:
> Generic resctrl exposes kernel-mode policy options via sysfs only after
> architectures indicate the set of supported policies using
> resctrl_set_kmode_support().
>
> On AMD systems, Privilege Level Zero Association (PLZA) provides two
> global assignment modes:
>
> - GLOBAL_ASSIGN_CTRL_INHERIT_MON_PER_CPU: Assigns the resource allocation
>    for kernel work; but monitoring is inherited from the user task.
> - GLOBAL_ASSIGN_CTRL_ASSIGN_MON_PER_CPU: Assigns a dedicated resource
>    allocation and monitoring for kernel work.
>
> Expose these modes during resource discovery when PLZA is available.
>
> Signed-off-by: Babu Moger <babu.moger@xxxxxxx>
> ---
> v4: New patch to set the supported features during arch init.
> ---
>   arch/x86/kernel/cpu/resctrl/core.c | 6 ++++++
>   1 file changed, 6 insertions(+)
>
> diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
> index 27e09cded829..df7e19fe6007 100644
> --- a/arch/x86/kernel/cpu/resctrl/core.c
> +++ b/arch/x86/kernel/cpu/resctrl/core.c
> @@ -962,6 +962,9 @@ static __init bool get_rdt_alloc_resources(void)
>   	if (get_slow_mem_config())
>   		ret = true;
>   
> +	if (ret && rdt_cpu_has(X86_FEATURE_PLZA))
> +		resctrl_set_kmode_support(GLOBAL_ASSIGN_CTRL_INHERIT_MON_PER_CPU);
> +
>   	return ret;
>   }
>   
> @@ -988,6 +991,9 @@ static __init bool get_rdt_mon_resources(void)
>   	if (!ret)
>   		return false;
>   
> +	if (rdt_cpu_has(X86_FEATURE_PLZA))
> +		resctrl_set_kmode_support(GLOBAL_ASSIGN_CTRL_ASSIGN_MON_PER_CPU);
> +
>   	return !rdt_get_l3_mon_config(r);
>   }
>   

Sashiko reported:

https://sashiko.dev/#/patchset/cover.1783461016.git.babu.moger%40amd.com?part=7

--- Patch [7]: [RESEND PATCH v4 07/15] x86/resctrl: Expose the supported 
PLZA kernel-mode policies during init ---
commit 6d029d053cc0d67da93c0b4ae5419b85cfaeb5aa
Author: Babu Moger <babu.moger@xxxxxxx>
x86/resctrl: Expose the supported PLZA kernel-mode policies during init
This commit updates the generic resctrl subsystem to expose PLZA
kernel-mode policies during resource discovery. When X86_FEATURE_PLZA
is present, it sets up support for GLOBAL_ASSIGN_CTRL_INHERIT_MON_PER_CPU
and GLOBAL_ASSIGN_CTRL_ASSIGN_MON_PER_CPU policies.
> diff --git a/arch/x86/kernel/cpu/resctrl/core.c 
b/arch/x86/kernel/cpu/resctrl/core.c
> index 27e09cded8299..df7e19fe60078 100644
> --- a/arch/x86/kernel/cpu/resctrl/core.c
> +++ b/arch/x86/kernel/cpu/resctrl/core.c
[ ... ]
> @@ -988,6 +991,9 @@ static __init bool get_rdt_mon_resources(void)
>  	if (!ret)
>  		return false;
>
> +	if (rdt_cpu_has(X86_FEATURE_PLZA))
> +		resctrl_set_kmode_support(GLOBAL_ASSIGN_CTRL_ASSIGN_MON_PER_CPU);
Is it possible for GLOBAL_ASSIGN_CTRL_ASSIGN_MON_PER_CPU to be advertised
here even if resource allocation features are disabled on the system?
Because this policy assigns a dedicated resource allocation for kernel work,
should this check verify rdt_alloc_capable before exposing the policy?
If allocation is disabled via kernel command-line parameters (like
rdt=!l3cat), but monitoring features are present, it seems like we might
expose an allocation-dependent policy to userspace incorrectly.
------------------------------------------------------------------------

PLZA is really meaningful when allocation is enabled. I think it would 
be better to check rdt_alloc_capable before enabling 
GLOBAL_ASSIGN_CTRL_ASSIGN_MON_PER_CPU. Will add the check.

if (rdt_alloc_capable && rdt_cpu_has(X86_FEATURE_PLZA)) 
resctrl_set_kmode_support(GLOBAL_ASSIGN_CTRL_ASSIGN_MON_PER_CPU);

Thanks
Babu