linux-kernel - RE: RE: [PATCH 1/2] x86/resctrl: Add io_alloc_min_cbm

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Message-ID: <SJ1PR11MB60836AB4270419338FBB4D1EFCCEA@SJ1PR11MB6083.namprd11.prod.outlook.com>
Date: Mon, 10 Nov 2025 19:56:32 +0000
From: "Luck, Tony" <tony.luck@...el.com>
To: Babu Moger <bmoger@....com>, Aaron Tomlin <atomlin@...mlin.com>
CC: "Chatre, Reinette" <reinette.chatre@...el.com>, "Dave.Martin@....com"
	<Dave.Martin@....com>, "james.morse@....com" <james.morse@....com>,
	"babu.moger@....com" <babu.moger@....com>, "tglx@...utronix.de"
	<tglx@...utronix.de>, "mingo@...hat.com" <mingo@...hat.com>, "bp@...en8.de"
	<bp@...en8.de>, "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: RE: [PATCH 1/2] x86/resctrl: Add io_alloc_min_cbm_all interface
 for CBM reset

> > # echo "L2:*=fff" > schemata
> >
> > would work,. But
> >
> > # echo "L2:*=ffff" > schemata
> >
> > would try to set unimplemented bits on some cores and would fail.
>
>
> I would consider this a user error, as the user is expected to know the
> supported value for the domain.
> This situation can occur even now — we simply report the error and exit.

Babu

Maybe it was a poor explanation on my part.

On a hybrid P-core/E-core system with different L2 cache topology schemata
may look like this (8 L2 domains of one type, 4 L2 domains of other type.

$ cat schemata
L2:0=ffff;1=ffff;2=ffff;3=ffff;4=ffff;5=ffff;6=ffff;7=ffff;8=7f;9=7f;10=7f;11=7f

The proposed wildcard syntax is only useful to set all domains to a value
that is legal for all domains. It cannot be used for the "reset back to defaults"
case because different domains have different defaults.

-Tony