Re: [PATCH v6 03/18] x86/resctrl: Add PLZA configuration definitions and data structures

[Reinette Chatre]

Hi Babu,

On 10/20/26 11:02 AM, Babu Moger wrote:
> On 10/19/26 11:16, Reinette Chatre wrote:
>> On 10/15/26 4:07 PM, Babu Moger wrote:
>>> On 10/15/26 09:22, Reinette Chatre wrote:
>>>> On 10/1/26 8:10 AM, Babu Moger wrote:
>>>>> Privilege Level Zero Association (PLZA) enables the kernel to switch to
>>>>> a different CLOSID and/or RMID than those used in user mode when entering
>>>>> kernel mode. CLOSID and RMID selection are independent and can be
>>>>> configured separately.
>>>>
>>>> That CLOSID and RMID are independent on x86 is clear to me. That they can be
>>>> configured separately is not clear to me in this context. What does this
>>>> "separate configuration" refer to? Looking at the later patches kernel
>>>> mode allocation and monitoring associations are always configured together
>>>> and written to same MSR at the same time?
>>>
>>>
>>> Good point. Its confusing.  What I meant is that allocation (CLOSID)
>>> and monitoring (RMID) associations are independent PLZA features and
>>> can be enabled independently in hardware. For example, a platform
>>> may enable kernel mode CLOSID association without enabling kernel
>>> mode RMID association, or vice versa.
>>>
>>> Yes. The kernel interface configures both together because they are carried in the same IA32_PQR_ASSOC MSR update.
>>>
>>> How about?
>>>
>>> Privilege Level Zero Association (PLZA) enables the kernel to use a
>>> different CLOSID and/or RMID in kernel mode than in user mode.
>>> Allocation (CLOSID) and monitoring (RMID) associations are
>>> independent PLZA features and may be enabled independently by
>>> hardware.
>>
>> Which CLOSID (MSR_IA32_PQR_PLZA_ASSOC.closid) and/or RMID (MSR_IA32_PQR_PLZA_ASSOC.rmid)
>> is used in kernel mode is different from whether the CLOSID (MSR_IA32_PQR_PLZA_ASSOC.closid_en)
>> and/or RMID (MSR_IA32_PQR_PLZA_ASSOC.rmid_en) is used in kernel mode.
>>
>> This series uses the high level "allocation and monitoring association" term
>> interchangeably for both of these meanings and doing so causes confusion.
>>
>> Please pick specific terms to refer to the CLOSID/RMID assigned to kernel mode vs
>> whether the CLOSID/RMID assignment is enabled and use it throughout the series.
>>
> 
> Another attempt to capture all the details:
> 
> "Privilege Level Zero Association (PLZA) enables the kernel to use a
> different CLOSID and/or RMID than those used in user mode when
> executing in kernel mode. CLOSID and RMID selection can be
> configured independently.

First sentence already contains "and/or" so I think the last sentence can just
be dropped.

> 
> The kernel mode CLOSID and RMID are specified by
> MSR_IA32_PQR_PLZA_ASSOC.closid and MSR_IA32_PQR_PLZA_ASSOC.rmid,
> respectively. plza_en activates the PLZA operation, while closid_en
> and rmid_en independently control whether the corresponding kernel
> mode CLOSID or RMID is used.

Please aim to provide higher level description than what can be seen
from the patch.

> 
> Make the register and its field layout available so kernel mode
> allocation (CLOSID) and monitoring (RMID) selection can be
> configured independently."

Apologies but I really do not understand why the "independently"
distinction is needed since these values are always programmed together.

Here is an attempt, but it is written without knowing which terms you have
picked to stick with:

  x86/resctrl: Add PLZA MSR definition and field layout

  Privilege Level Zero Association (PLZA) enables a CPU to use a different
  CLOSID and/or RMID when executing in kernel mode than those used in user
  mode.

  PLZA is programmed per CPU through MSR_IA32_PQR_PLZA_ASSOC, which contains
  the kernel mode CLOSID and RMID, an enable bit for each, and a bit that
  enables PLZA on the CPU. All fields except the PLZA enable bit must be
  programmed identically on every CPU in a domain.

  Make the register and its field layout available in preparation for resctrl
  support of kernel mode allocation and monitoring.

Reinette