Re: [RESEND PATCH v4 01/15] x86/resctrl: Support Privilege Level Zero Association (PLZA)

[Reinette Chatre]

Hi Babu,

Could you please switch subject prefix to be "x86/cpufeatures:"?

On 7/7/26 2:50 PM, Babu Moger wrote:
> Customers have identified an issue while using the QoS resource control
> feature. If memory bandwidth associated with a CLOSID is aggressively
> throttled, and a task with that CLOSID moves into kernel mode, the kernel
> operations are also aggressively throttled. This can stall forward progress
> and eventually degrade overall system performance.
> 
> AMD hardware supports a feature Privilege Level Zero Association (PLZA),
> which allows the CPU's CLOSID association to be changed during the
> transition from user mode to kernel mode. This allows the kernel to run
> using a different CLOSID than user space, which can improve system
> performance in certain scenarios.

How should user interpret "in certain scenarios"? Could the scenarios 
be highlighted ... or maybe just drop this since it is already preceded
with "can improve system performance".

This description implies that PLZA is a feature that is only related
to memory bandwidth allocation where it can be beneficial. As I understand
the feature impacts all allocation features concurrently, including cache
allocation where it should be used with care?

If the goal is to describe all the feature capabilities then it may be
worth a mention of the impact/usage related to monitoring?

> 
> The feature is detected via CPUID_Fn80000020_EBX_x00 [Bit 9]:
> Privilege Level Zero Association (PLZA).
> 
> The PLZA feature details are documented in [1] available from [2].
> 
> [1] AMD64 Zen6 Platform Quality of Service (PQOS) Extensions:
>     Publication # 69193 Revision: 1.00, Issue Date: March 2026
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537 # [2]
> Signed-off-by: Babu Moger <babu.moger@xxxxxxx>

Please follow "Ordering of commit tags" found in Documentation/process/maintainer-tip.rst.

Reinette