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

[Babu Moger <babu.moger@amd.com>]

Hi Reinette,

On 8/10/26 21:51, Reinette Chatre wrote:
> Hi Babu,
> 
> Could you please switch subject prefix to be "x86/cpufeatures:"?

Yes.

> 
> 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".

Will remove "in certain scenarios".

> 
> 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?

Yes. Let me add text about CAT.

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

Yes. Need to mention monitoring also.

> 
>>
>> 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@amd.com>
> 
> Please follow "Ordering of commit tags" found in Documentation/process/maintainer-tip.rst.
> 
Yes. Link should be the last one.

Thanks
Babu