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

[Babu Moger]

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