linux-kernel - Re: [LSF/MM] CXL Boot to Bash - Section 1: BIOS, EFI, and Early Boot

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Message-ID: <Z9MT1fIKu0v1r9ss@gourry-fedora-PF4VCD3F>
Date: Thu, 13 Mar 2025 13:20:21 -0400
From: Gregory Price <gourry@...rry.net>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc: lsf-pc@...ts.linux-foundation.org, linux-mm@...ck.org,
	linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [LSF/MM] CXL Boot to Bash - Section 1: BIOS, EFI, and Early Boot

On Thu, Mar 13, 2025 at 04:12:26PM +0000, Jonathan Cameron wrote:
> On Mon, 3 Mar 2025 19:32:43 -0500
> Gregory Price <gourry@...rry.net> wrote:
> 
> > 
> > SRAT entries are optional, CFMWS are required for each host bridge.
> 
> They aren't required for each HB.  You could have multiple host bridge and one CFMWS
> as long as you have decided to only support interleave.
> I would only expect to see this where the bios is instantiating CFMWS
> entries to match a specific locked down config though.
>

The further I dived into this, the more I realized CFMWS are the
opposite of required lol.  Platform vendors can kind of do whatever they
want here.

I'll be taking another pass at this section since i've done more diving
in to write the interleave section.  I probably got a handful of
comments here subtly wrong.

> > 
> > If SRAT entries are present, one NUMA node is created for each detected
> > proximity domain in the SRAT. Additional NUMA nodes are created for each
> > CFMWS without a matching SRAT entry.
> 
> Don't forget the fun of CFMWS covering multiple SRAT entries (I think
> we just go with the first one?)
>

Oh yeah, I guess that's technically possible. And technically each SRAT
could have a different proximity domain, because you know - value.

The dance between CFMWS and SRAT is quite intricate isn't it.

> > 
> > CFMWS describes host-bridge information, and so if SRAT is missing - all
> > devices behind the host bridge will become naturally associated with the
> > same NUMA node.
> 
> I wouldn't go with naturally for the reason below.  It happens, but maybe
> not natural :)
>

Yeah as above, I got this subtly wrong.  Thanks for the notes.

~Gregory