Re: [RFC PATCH v2 0/8] mm: Hot page tracking and promotion infrastructure

[Gregory Price <gourry@gourry.net>]

On Fri, Oct 17, 2025 at 03:36:13PM +0100, Jonathan Cameron wrote:
> On Fri, 17 Oct 2025 10:15:57 -0400
> Gregory Price <gourry@gourry.net> wrote:
> > 
> > Essentially the platform needs to allow a single device to expose
> > multiple numa nodes based on different expected performance.  From
> > those ranges.  Then software needs to program the HDM decoders
> > appropriately.
> 
> It's a bit 'fuzzy' to justify but maybe (for CXL) a CFWMS flag (so CEDT
> as you mention) to say this host memory region may be backed by
> compressed memory?
>
> Might be able to justify it from spec point of view by arguing that
> compression is a QoS related characteristic. Always possible host
> hardware will want to handle it differently before it even hits the
> bus even if it's just a case throttling writing differently.
>

That's a Consortium discussion to have (and I am not of the
consortium :P), but yeah you could do it that way.

More generally could have a "Not-for-general-consumption bit" instead
of specifically a compressed bit.  Maybe both a "No-Consume" and a
"Special Node" bit would be useful separately.

Of course then platforms need to be made to understand all these:

"No-Consume" -> force EFI_MEMORY_SP or leave it reserved
"Special Node" -> allocate its own PXM / Provide discrete CFMWS

Naming obviously non-instructive here, may as well call them Nancy and
Bob bits.

> That then ends up in it's own NUMA node.  Whether we take on the
> splitting CFMWS entries into multiple NUMA nodes depending on what
> backing devices end up in them is something we kicked into the long
> grass originally, but that can definitely be revisited.  That
> doesn't matter for initial support of compressed memory though if
> we can do it via a seperate CXL Fixed Memory Window Structure (CFMWS)
> in CEDT.
>

This is the way I would initially approach it tbh - but i'm also not a
hardware/firmware person, so i don't know exactly what bits a device
would set to tell BIOS/EFI "Hey, give this chunk its own CFMWS", or if
that lies solely with BIOS/EFI.

~Gregory