Re: [PATCH] ACPI: NUMA: Only parse CFMWS at boot when CXL_ACPI is on

[Gregory Price <gourry@gourry.net>]

On Wed, Mar 04, 2026 at 03:20:11PM -0800, Alison Schofield wrote:
> On Wed, Mar 04, 2026 at 05:33:26PM -0500, Gregory Price wrote:
> > On Thu, Mar 05, 2026 at 10:33:42AM +1300, Kai Huang wrote:
> > > Increasing the 'nr_node_ids' has side effects.  For instance, it is
> > > widely used by the kernel for "highest possible NUMA node" based memory
> > > allocations.  It also impacts userspace ABIs, e.g., some NUMA memory
> > > related system calls such as 'get_mempolicy' which requires 'maxnode'
> > > not being smaller than the 'nr_node_ids'.
> > > 
> 
> > 
> > Is this a Linux issue or a Firmware issue?
> 
> IIUC BIOS creates the CEDT based on the hardware it 'sees' as present.
> 
> This patch is describing the case (weird as it seems to me) where we
> then boot a system with ACPI and NUMA enabled but CXL_ACPI disabled.
> 
> So, I don't think we can blame BIOS.
> 
> > 
> > Is GNR exporting more CFMWS than it should?
> Not sure of any limits on flavors of CFMWS's a BIOS can offer.
> If BIOS can carve out a window, it can create a CFMWS.
> Not clear how that matters to the issue.
> 
> > 
> > Is your SRAT missing entries for CFMWS that are otherwise present?
> > 
> > Are the CFMWS empty? (is that even valid)
> 
> Why this line of questioning ;)  I see the problem as a bit simpler.
> We have other code that tells us if the CFMWS's are valid, etc, but
> the point here is, we are not going to use these CFMWS's so stop
> the parse as early as possible, like right here as Kai has done.
> 

Mostly i'm wondering if this issue should be dealt with in the acpi code
or if the issue is that we just don't want to figure out how to
lazy-create these things instead of always creating them at __init.

it does seem rational to build out support for CEDT entries if CXL_ACPI
is built out, but this also means you can't otherwise load modules that
would have made use of this information.

This basically says if specifically CXL_ACPI is built out, the NUMA
structure is forever lost - even though it's accurately described by
BIOS.  Maybe that's a rational decision, just kind of prodding a bit.

~Gregory