Re: [PATCH v3 04/12] x86,fs/resctrl: Program PLZA through kmode arch hooks

[Moger, Babu]

Hi Reinette,

On 6/16/2026 6:33 PM, Reinette Chatre wrote:
> Hi Babu,
>
> On 4/30/26 4:24 PM, Babu Moger wrote:
> > AMD Privilege Level Zero Association (PLZA) exposes kernel CLOSID/RMID
> > association through MSR_IA32_PQR_PLZA_ASSOC.  Generic resctrl already
> > tracks supported and effective kernel-mode policy in struct
> > resctrl_kmode_cfg, but the architecture layer needs a callable entry point
> > that can push those values into per-CPU hardware on a chosen CPU mask.
> >
> > Declare resctrl_arch_configure_kmode() in linux/resctrl.h with kernel-doc.
> > Implement it on x86: add an SMP callback that writes
> > MSR_IA32_PQR_PLZA_ASSOC on each targeted CPU, and use on_each_cpu_mask()
> > for the broadcast.
>
> Above is clear from the patch. Please start with focus on why this patch is
> needed.
>
> > The hook is unused in this patch; later patches in the series wire it into
>
> Similar to previous work: write changelog in imperative tone and do not
> refer to patches in series but instead let each patch stand on its own.

Will rewrite the changelog.

> > generic resctrl when an effective kernel-mode policy is selected or a CPU
> > mask changes.
> >
> > Signed-off-by: Babu Moger <babu.moger@xxxxxxx>
> > ---
>
>
> > ---
> >   arch/x86/kernel/cpu/resctrl/ctrlmondata.c | 35 +++++++++++++++++++++++
> >   include/linux/resctrl.h                   | 10 +++++++
> >   2 files changed, 45 insertions(+)
> >
> > diff --git a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> > index b20e705606b8..68f1cf503904 100644
> > --- a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> > +++ b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> > @@ -131,3 +131,38 @@ int resctrl_arch_io_alloc_enable(struct rdt_resource *r, bool enable)
> >   
> >   	return 0;
> >   }
> > +
> > +/*
> > + * SMP call-function callback: each CPU writes its own MSR_IA32_PQR_PLZA_ASSOC
> > + * (AMD PLZA).  Invoked via on_each_cpu_mask() with wait=1 so the on-stack
> > + * union pointed at by @arg is safe.
> > + */
> > +static void resctrl_kmode_set_one_amd(void *arg)
> > +{
> > +	union msr_pqr_plza_assoc *plza = arg;
> > +
> > +	wrmsrl(MSR_IA32_PQR_PLZA_ASSOC, plza->full);
>
> fyi ...
> commit 2232959db26d ("x86/msr: Switch wrmsrl() users to wrmsrq()")
> commit b5884070f9da ("x86/msr: Remove wrmsrl()")

Yes. Saw that. Will change it to wrmsrq.

> > +}
> > +
> > +/**
> > + * resctrl_arch_configure_kmode() - x86/AMD: program PLZA MSR on a CPU subset
> > + * @cpu_mask:	CPUs to receive the update (see on_each_cpu_mask() for online subset).
>
> Why is the caveat added? Will resctrl ever provide offline CPUs in the mask?

No. Offline CPUs will not be provided. I am not sure why I added that 
caveat. Probably came from AI review. Will remove.

> > + * @closid:	CLOSID field written into the MSR with CLOSID_EN set.
> > + * @rmid:	RMID field written into the MSR with RMID_EN set.
> > + * @enable:	Value for the PLZA_EN split field.
>
> Please describe the meaning of the fields instead the mechanics of the code
> that are obvious.

ok.

> > + *
> > + * Context: Do not call with IRQs off or from IRQ context except as allowed for
> > + * on_each_cpu_mask(); see kernel/smp.c.
>
> Why is this context caveat needed?

Again, Probably came from AI review. Does not look relevant. Will remove.

> > + */
> > +void resctrl_arch_configure_kmode(cpumask_var_t cpu_mask, u32 closid, u32 rmid, bool enable)
>
> Please replace "cpumask_var_t cpu_mask" with "const struct cpumask *cpu_mask".

Sure.

> > +{
> > +	union msr_pqr_plza_assoc plza = { 0 };
> > +
> > +	plza.split.rmid = rmid;
> > +	plza.split.rmid_en = 1;
> > +	plza.split.closid = closid;
> > +	plza.split.closid_en = 1;
> > +	plza.split.plza_en = enable;
> > +
> > +	on_each_cpu_mask(cpu_mask, resctrl_kmode_set_one_amd, &plza, 1);
> > +}
>
> function self has been discussed already
>
> > diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
> > index ce28418df00f..570918e57e24 100644
> > --- a/include/linux/resctrl.h
> > +++ b/include/linux/resctrl.h
> > @@ -712,6 +712,16 @@ bool resctrl_arch_get_io_alloc_enabled(struct rdt_resource *r);
> >    */
> >   void resctrl_arch_get_kmode_support(struct resctrl_kmode_cfg *kcfg);
> >   
> > +/**
> > + * resctrl_arch_configure_kmode() - Program MSR_IA32_PQR_PLZA_ASSOC on CPUs in @cpu_mask
> > + * @cpu_mask:	Target CPUs; on_each_cpu_mask() applies the callback on the online subset.
> > + * @closid:	CLOSID written to the MSR with CLOSID_EN set.
> > + * @rmid:	RMID written to the MSR with RMID_EN set.
> > + * @enable:	PLZA_EN field value for this update.
>
> This is a resctrl fs API - please replace all the AMD architecture specific implementation details
> with what the parameters actually mean/represent.

Sure. Will rewrite it.

Thanks

Babu