linux-kernel - RE: [PATCH] fs/resctrl,x86/resctrl: Factor mba rounding to be per-arch

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Message-ID: <SJ1PR11MB6083F946E57D3C1723F341EDFCE7A@SJ1PR11MB6083.namprd11.prod.outlook.com>
Date: Thu, 2 Oct 2025 16:43:46 +0000
From: "Luck, Tony" <tony.luck@...el.com>
To: Dave Martin <Dave.Martin@....com>, "Chen, Yu C" <yu.c.chen@...el.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Chatre,
 Reinette" <reinette.chatre@...el.com>, James Morse <james.morse@....com>,
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>, Jonathan Corbet <corbet@....net>,
	"x86@...nel.org" <x86@...nel.org>, "linux-doc@...r.kernel.org"
	<linux-doc@...r.kernel.org>
Subject: RE: [PATCH] fs/resctrl,x86/resctrl: Factor mba rounding to be
 per-arch

> > > So, if ACPI says Q=387, that's the denominator we advertise.
> > >
> > > Does that sound right?
> > >
> >
> > Yes, it makes sense, the denominator is the "scale" in your example.
>
> Thanks for confirming that.
>
> > > Question: is this a global parameter, or per-CPU?
> > >
> >
> > It should be a global setting for all the MBA Register Blocks.
>
> That's good -- since resctrl resource controls are not per-CPU,
> exposing the exact hardware resolution won't work unless the value
> is scaled identically for all CPUs.

The RDT architecture spec says there is a separate MARC table that
describes each instance on an L3 cache.

So in theory there could be different "Q" values for each. I'm chatting
with the architects to point out that would be bad, and they shouldn't
build something that has different "Q" values on the same system.

-Tony