Re: [RFC PATCH 13/19] x86/resctrl: Add PLZA state tracking and context switch handling

[Ben Horgan <ben.horgan@arm.com>]

Hi Stephane,

On 2/18/26 09:35, Ben Horgan wrote:
> Hi Stephane,
> 
> On 2/18/26 06:22, Stephane Eranian wrote:
>> On Tue, Feb 17, 2026 at 7:56 AM Ben Horgan <ben.horgan@arm.com> wrote:
>>>
>>> Hi Babu,
>>>
>>> On 2/16/26 22:52, Moger, Babu wrote:
>>>> Hi Ben,
>>>>
>>>> On 2/16/2026 9:41 AM, Ben Horgan wrote:
>>>>> Hi Babu, Reinette,
>>>>>
>>>>> On 2/14/26 00:10, Reinette Chatre wrote:
>>>>>> Hi Babu,
>>>>>>
>>>>>> On 2/13/26 8:37 AM, Moger, Babu wrote:
>>>>>>> Hi Reinette,
>>>>>>>
>>>>>>> On 2/10/2026 10:17 AM, Reinette Chatre wrote:
>>>>>>>> Hi Babu,
>>>>>>>>
>>>>>>>> On 1/28/26 9:44 AM, Moger, Babu wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On 1/28/2026 11:41 AM, Moger, Babu wrote:
>>>>>>>>>>> On Wed, Jan 28, 2026 at 10:01:39AM -0600, Moger, Babu wrote:
>>>>>>>>>>>> On 1/27/2026 4:30 PM, Luck, Tony wrote:
>>>>>>>>>>> Babu,
>>>>>>>>>>>
>>>>>>>>>>> I've read a bit more of the code now and I think I understand more.
>>>>>>>>>>>
>>>>>>>>>>> Some useful additions to your explanation.
>>>>>>>>>>>
>>>>>>>>>>> 1) Only one CTRL group can be marked as PLZA
>>>>>>>>>>
>>>>>>>>>> Yes. Correct.
>>>>>>>>
>>>>>>>> Why limit it to one CTRL_MON group and why not support it for MON
>>>>>>>> groups?
>>>>>>>
>>>>>>> There can be only one PLZA configuration in a system. The values in
>>>>>>> the MSR_IA32_PQR_PLZA_ASSOC register (RMID, RMID_EN, CLOSID,
>>>>>>> CLOSID_EN) must be identical across all logical processors. The only
>>>>>>> field that may differ is PLZA_EN.
>>>>>
>>>>> Does this have any effect on hypervisors?
>>>>
>>>> Because hypervisor runs at CPL0, there could be some use case. I have
>>>> not completely understood that part.
>>>>
>>>>>
>>>>>>
>>>>>> ah - this is a significant part that I missed. Since this is a per-
>>>>>> CPU register it seems
>>>>>
>>>>> I also missed that.
>>>>>
>>>>>> to have the ability for expanded use in the future where different
>>>>>> CLOSID and RMID may be
>>>>>> written to it? Is PLZA leaving room for such future enhancement or
>>>>>> does the spec contain
>>>>>> the text that state "The values in the MSR_IA32_PQR_PLZA_ASSOC
>>>>>> register (RMID, RMID_EN,
>>>>>> CLOSID, CLOSID_EN) must be identical across all logical processors."?
>>>>>> That is, "forever
>>>>>> and always"?
>>>>>>
>>>>>> If I understand correctly MPAM could have different PARTID and PMG
>>>>>> for kernel use so we
>>>>>> need to consider these different architectural behaviors.
>>>>>
>>>>> Yes, MPAM has a per-cpu register MPAM1_EL1.
>>>>>
>>>>
>>>> oh ok.
>>>>
>>>>>>
>>>>>>> I was initially unsure which RMID should be used when PLZA is
>>>>>>> enabled on MON groups.
>>>>>>>
>>>>>>> After re-evaluating, enabling PLZA on MON groups is still feasible:
>>>>>>>
>>>>>>> 1. Only one group in the system can have PLZA enabled.
>>>>>>> 2. If PLZA is enabled on CTRL_MON group then we cannot enable PLZA
>>>>>>> on MON group.
>>>>>>> 3. If PLZA is enabled on the CTRL_MON group, then the CLOSID and
>>>>>>> RMID of the CTRL_MON group can be written.
>>>>>>> 4. If PLZA is enabled on a MON group, then the CLOSID of the
>>>>>>> CTRL_MON group can be used, while the RMID of the MON group can be
>>>>>>> written.
>>>>>
>>>>> Given that CLOSID and RMID are fixed once in the PLZA configuration
>>>>> could this be simplified by just assuming they have the values of the
>>>>> default group, CLOSID=0 and RMID=0 and let the user base there
>>>>> configuration on that?
>>>>>
>>>>
>>>> I didn't understand this question. There are 16 CLOSIDs and 1024 RMIDs.
>>>> We can use any one of these to enable PLZA.  It is not fixed in that sense.
>>>
>>> Sorry, I wasn't clear. What I'm trying to understand is what you gain by
>>> this flexibility. Given that the values CLOSID and the RMID are just
>>> identifiers within the hardware and have only the meaning they are given
>>> by the grouping and controls/monitors set up by resctrl (or any other
>>> software interface) would you lose anything by just saying the PLZA
>>> group has CLOSID=0 and RMID=0. Is there value in changing the PLZA
>>> CLOSID and RMID or can the same effect happen by just changing the
>>> resctrl configuration?
>>>
>> Not quite.
>> When you enter the kernel, you want to run unthrottled to avoid
>> priority inversion situations.

In cases where you want to reserve a cache region for one program you
may want to avoid giving the kernel access to that region so it doesn't
pollute it.

>> But at the same time, you still want to be able to monitor the
>> bandwidth for your thread or job, i..e, keep the same
>> RMID you have in user space.
> 
> Thanks for sharing your usecase.
> 
>>
>> The kernel is by construction shared by all threads running in the
>> system. It should run unrestricted or with the
>> bandwidth allocated to the highest priority tasks.
>>
>> PLZA should not change the RMID at all.
> 
> Would the above with RMID_EN=0 give you this usecase?
> 
> Unfortunately, this isn't possible when rmid/pmg is scoped to
> closid/partid as is the case in MPAM, i.e. the monitors require a match
> on the pair (closid, partid). Hence, I think we need to support the case
> where both RMID and CLOSID change.
> 


Thanks,

Ben