Re: [PATCH v5 01/16] x86/cpufeatures: Support Privilege Level Zero Association (PLZA)

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

Hi Reinette,

On 9/17/26 10:04, Reinette Chatre wrote:
> Hi Babu,
> 
> On 9/16/26 1:45 PM, Babu Moger wrote:
>> On 9/16/26 00:12, Reinette Chatre wrote:
>>> On 8/26/26 12:32 PM, Babu Moger wrote:
> 
> 
> ...
> 
>>
>>>
>>>>
>>>> PLZA also applies to cache allocation and monitoring features and provides
>>>> flexibility to manage allocation and monitoring associations independently.
>>>
>>> This reads like an afterthought. Are users expected to use cache allocation
>>> while PLZA is enabled? Since only one resource group can be assigned I expect
>>> there to be quite a few sharp corners when both are in use. Kernel work done
>>> on behalf of high as well as low priority work will use the same cache allocations
>>> and thus neither could actually get full benefit from having dedicated cache
>>> portions?
>>
>> The best-known use case today is to leave both cache allocation and
>> memory bandwidth allocation at their default (maximum) settings and
>> enable PLZA on all CPUs.
> 
> Interesting. This sounds like all kernel work can access the entire cache
> and thus evict data from any user space tasks that may have used cache allocation
> to avoid interference.
> 
>>
>> Users can, however, configure both cache allocation and memory bandwidth limits. In those cases, there can be corner-case behaviors and trade-offs.
>>
>> If you think the current wording is misleading, I can drop that text and rephrase it along these lines to include both cache and memory.
> 
> No, the wording is not misleading. The description just focuses on memory bandwidth
> allocation leaving impression that the feature only applies to that resource when it
> applies to all resources.
> 
>>
>> "When cache and/or 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 the Privilege Level Zero Association (PLZA)
>> feature, which allows the CPU's CLOSID and/or RMID association to be
>> changed during transitions from user mode to kernel mode. This
>> enables the kernel to run with a different CLOSID and/or RMID than
>> user space, improving system performance and/or allowing kernel work
>> to be monitored separately.">
> 
> I think what you have for memory bandwidth sounds good. I do not think
> the cache allocation and memory bandwidth usage descriptions should be
> mixed since the implications are so different. Could you please just
> add a snippet that highlights since the feature assigns CLOSID/RMID to
> kernel work it influences allocation and monitoring of all resources,
> not just memory bandwidth?
> 
Yes. Sure.
Thanks
Babu