linux-kernel - Re: [PATCH v2 1/1] mm: numa_memblks: Identify the accurate NUMA ID of CFMW

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Message-ID: <aYYOZ1TK5dpX_h_Q@gourry-fedora-PF4VCD3F>
Date: Fri, 6 Feb 2026 10:53:11 -0500
From: Gregory Price <gourry@...rry.net>
To: Jonathan Cameron <jonathan.cameron@...wei.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Cui Chao <cuichao1753@...tium.com.cn>, dan.j.williams@...el.com,
	Mike Rapoport <rppt@...nel.org>,
	Wang Yinfeng <wangyinfeng@...tium.com.cn>,
	linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, qemu-devel@...gnu.org,
	"David Hildenbrand (Arm)" <david@...nel.org>
Subject: Re: [PATCH v2 1/1] mm: numa_memblks: Identify the accurate NUMA ID
 of CFMW

On Fri, Feb 06, 2026 at 03:09:41PM +0000, Jonathan Cameron wrote:
> On Fri, 6 Feb 2026 08:31:09 -0500
> Gregory Price <gourry@...rry.net> wrote:
> 
> Now a fun corner is that a node isn't created unless there is something
> in it - the whole SRAT is the source of truth for what nodes exist
> - so we need 'something' in it - a cpu will do, or a GI, probably a GP.
> Otherwise memory ends up in node0.  However, fallback lists etc happen
> as normal when first mem in a node is added.
> 
...
> For now I 'suspect' we could hack things to provide lots of waiting numa nodes
> and merrily assign HPA into them as we like whatever SRAT provides
> in the way of 'hints' :) 
> 

look at ACPI MSCT - "Maximum Proximity Domain Information Structure" ;]

I don't remember reading anything in the ACPI spec that says something
has to be ON any of these PXMs for it to be accounted for in the MSCT.

Platforms can just say "Reserve that many Nodes".

(Linux does not read this value, and on my existing systems, this number
always reflects the number of actually present PXMs)

---

We probably want to ignore that and just add this:

CONFIG_ACPI_NUMA_NODES_PER_CFMWS
    int
    range 1 4
    help
        This option determines the number of NUMA nodes that will be
	added for each CEDT CFMWS entry.

	By default ACPI reserves 1 per unique PXM entry in the SRAT,
	or 1 for a CXL Fixed Memory Window without SRAT mappings.

	This will reserve up to N nodes per CEDT entry, even if that
	CEDT has one or more SRAT entries.

then in the acpi/numa/srat.c code that parses srat/cedt, just track
the number of nodes over a CEDT range.

for each srat:
   account_unique_pxm(pxm, srat_range)

for each cedt:
   nr_nodes = unique_pxms(cedt_range)
   while (nr_nodes < CONFIG_ACPI_NUMA_NODES_PER_CFMWS)
      node = acpi_map_pxm_to_node(*fake_pxm++);
      if (node == NUMA_NO_NODE):
      	err("Unable to reserve additional nodes for CXL windows")
	break;
      node_set(node, numa_nodes_parsed);
      nr_nodes++

This should fall out cleanly.

The additional nodes won't be associated with anything, but could be
used for hotplug - I imagine.

~Gregory