Re: [PATCH v3 01/12] x86/resctrl: Support Privilege-Level Zero Association (PLZA)

[Babu Moger]

Hi Reinette,


On 6/16/26 19:00, Reinette Chatre wrote:
> Hi Babu,
>
> On 6/12/26 9:56 AM, Moger, Babu wrote:
> > Hi Reinette,
> >
> > On 6/11/2026 6:23 PM, Reinette Chatre wrote:
> > > Hi Babu,
> > >
> > > On 4/30/26 4:24 PM, Babu Moger wrote:
> > > > Customers have identified an issue while using the QoS resource Control
> > >
> > > "Control" -> "control"?
> >
> > ack
> >
> > > > feature. If a memory bandwidth associated with a CLOSID is aggressively
> > >
> > > "a memory bandwidth" -> "memory bandwidth"?
> >
> > ack.
> >
> > > > throttled, and it moves into Kernel mode, the Kernel operations are also
> > >
> > > What does "it" refer to here? From text it seems to be the "CLOSID" but that
> > > does not sound right? Should "it" instead be something like "a task with that
> > > CLOSID"?
> >
> > sure.
> >
> > > "Kernel" -> "kernel"?
> >
> > ack.
> > > > aggressively throttled. This can stall forward progress and eventually
> > > > degrade overall system performance. AMD hardware supports a feature
> > > > Privilege-Level Zero Association (PLZA) to change the association of the
> > > > thread as soon as it begins executing.
> > >
> > > "change the association of the thread as soon as it begins executing." I am
> > > not able to parse this.
> >
> > How about ?
> >
> > 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)
> > to change the CPU association at the user-to-kernel transition, so the kernel execution can use a different association than user mode.
>
> "change the CPU association at the user-to-kernel transition" -> What is this
> trying to describe? CPU association of what?
>
> "a different association"? What does this mean?

Will change it to:

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 enables the kernel to 
operate with a different CLOSID than the user mode.


> > Privilege-Level Zero Association (PLZA) allows the user to specify a> CLOSID and/or RMID associated with execution in Privilege-Level
> > Zero. When enabled on a CPU, as the CPU enters Privilege-Level Zero,
> > allocation and monitoring for that CPU will be associated with the
> > PLZA CLOSID and/or RMID. Otherwise, the CPU will be associated with
> > the CLOSID and RMID given by PQR_ASSOC.
>
>
> Sounds like this is vague because MSR_IA32_PQR_PLZA_ASSOC has not been
> introduced yet. Could it help to introduce MSR_IA32_PQR_PLZA_ASSOC as
> part of this patch and then the changelog can be specific about PLZA
> feature introducing this new MSR and how it complements MSR_IA32_PQR_ASSOC?

Its probably better to remove the second paragraph. This text can go 
with the patch which introduces MSR_IA32_PQR_PLZA_ASSOC.

With splitting the patch, this will only have cpufeatures changes.

> ...
>
> > > >    Documentation/admin-guide/kernel-parameters.txt | 2 +-
> > > >    arch/x86/include/asm/cpufeatures.h              | 1 +
> > > >    arch/x86/kernel/cpu/resctrl/core.c              | 2 ++
> > > >    arch/x86/kernel/cpu/scattered.c                 | 1 +
> > >
> > > Please split changes to other subsystems and make these changes
> > > obvious with their own subject prefix to avoid sneaking changes into
> > > other subsystems via resctrl.
> >
> > Ok. Will be two patches.
> > 1. For Documentation/admin-guide/kernel-parameters.txt
> > 2.  arch/x86/include/asm/cpufeatures.h
> >      arch/x86/kernel/cpu/resctrl/core.c
> >      arch/x86/kernel/cpu/scattered.c
>
> The resctrl changes found in (2) would be documented in (1)? That does not
> look right. Why not just split the resctrl changes from the cpufeatures changes?
> This would be similar to how you did ABMC enabling.

Sounds good.

Thanks
Babu