linux-kernel - Re: [RFC PATCH 14/19] x86,fs/resctrl: Add the functionality to configure PLZA

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Message-ID: <b5741094-fbb9-4dc0-b3a4-bb66073344ae@amd.com>
Date: Thu, 29 Jan 2026 13:53:49 -0600
From: Babu Moger <bmoger@....com>
To: "Luck, Tony" <tony.luck@...el.com>, Babu Moger <babu.moger@....com>
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Subject: Re: [RFC PATCH 14/19] x86,fs/resctrl: Add the functionality to
 configure PLZA

Hi Tony,


On 1/29/26 13:13, Luck, Tony wrote:
> On Wed, Jan 21, 2026 at 03:12:52PM -0600, Babu Moger wrote:
>> Privilege Level Zero Association (PLZA) is configured by writing to
>> MSR_IA32_PQR_PLZA_ASSOC. PLZA is disabled by default on all logical
>> processors in the QOS Domain. System software must follow the following
>> sequence.
>>
>> 1. Set the closid, closid_en, rmid and rmid_en fields of
>> MSR_IA32_PQR_PLZA_ASSOC to the desired configuration on all logical
>> processors in the QOS Domain.
>>
>> 2. Set MSR_IA32_PQR_PLZA_ASSOC[PLZA_EN]=1 for
>> all logical processors in the QOS domain where PLZA should be enabled.
>>
>> MSR_IA32_PQR_PLZA_ASSOC[PLZA_EN] may have a different value on every
>> logical processor in the QOS domain. The system software should perform
>> this as a read-modify-write to avoid changing the value of closid_en,
>> closid, rmid_en, and rmid fields of MSR_IA32_PQR_PLZA_ASSOC.
> Architecturally this is true. But in the implementation for resctrl
> there is only one PLZA group. So the CLOSID and RMID fields are
> identical on every logical processor. The only changing bit is the
> PLZA_EN.
Correct.
>
> The code could be simpler if you just maintained a single global
> with the CLOSID/RMID bits initialized by resctrl_arch_plza_setup().
>
> union qos_pqr_plza_assoc plza_value; // needs a better name
>

Yea. That is a good point.   We don't have to store CLOSID/RMID in

per-CPU state. Will do those changes in my next revision.


> Change the PLZA_EN define to be
>
> #define PLZA_EN	BIT_ULL(63)
>
> and then the hook into the __resctrl_sched_in() becomes:
>
>
> 	if (static_branch_likely(&rdt_plza_enable_key)) {
> 		u32 plza = READ_ONCE(state->default_plza); // note, moved this inside the static branch
> 		tmp = READ_ONCE(tsk->plza);
> 		if (tmp)
> 			plza = tmp;
>
> 		if (plza != state->cur_plza) {
> 			state->cur_plza = plza;
> 			wrmsrq(MSR_IA32_PQR_PLZA_ASSOC,
> 			       (plza ? PLZA_EN : 0) | plza_value.full);
> 		}
> 	}
>
> [Earlier e-mail about clearing the high half of MSR_IA32_PQR_PLZA_ASSOC
> was wrong. My debug trace printed the wrong value. The argument to the
> wrmsrl() is correct].


Got it. Thanks

Babu