linux-kernel - Re: [LSF/MM] CXL Boot to Bash - Section 0a: CFMWS and NUMA Flexiblity

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Message-ID: <Z9MhVZmFVTPpuRe1@gourry-fedora-PF4VCD3F>
Date: Thu, 13 Mar 2025 14:17:57 -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 0a: CFMWS and NUMA Flexiblity

On Thu, Mar 13, 2025 at 05:20:04PM +0000, Jonathan Cameron wrote:
> Gregory Price <gourry@...rry.net> wrote:
> 
> > -------------------------------
> > One 2GB Device, Multiple CFMWS.
> > -------------------------------
> > Lets imagine we have one 2GB device attached to a host bridge.
> > 
> > In this example, the device hosts 2GB of persistent memory - but we
> > might want the flexibility to map capacity as volatile or persistent.
> 
> Fairly sure we block persistent in a volatile CFMWS in the kernel.
> Any bios actually does this?
> 
> You might have a variable partition device but I thought in kernel at
> least we decided that no one was building that crazy?
> 

This was an example I pulled from Dan's notes elsewhere (i think).

I was unaware that we blocked mapping persistent as volatile.  I was
working off the assumption that could be flexible mapped similar to...
er... older, non-cxl hardware... cough.

> Maybe a QoS split is a better example to motivate one range, two places?
> 

That probably makes sense? 

> > -------------------------------------------------------------
> > Two Devices On One Host Bridge - With and Without Interleave.
> > -------------------------------------------------------------
> > What if we wanted some capacity on each endpoint hosted on its own NUMA
> > node, and wanted to interleave a portion of each device capacity?
> 
> If anyone hits the lock on commit (i.e. annoying BIOS) the ordering
> checks on HPA kick in here and restrict flexibility a lot
> (assuming I understand them correctly that is)
> 
> This is a good illustration of why we should at some point revisit
> multiple NUMA nodes per CFMWS.  We have to burn SPA space just
> to get nodes.  From a spec point of view all that is needed here
> is a single CFMWS. 
>

Along with the above note, and as mentioned on discord, I think this
whole section naturally evolves into a library of "Sane configurations"
and "We promise nothing for `reasons`" configurations.

Maybe that turns into a kernel doc section that requires updating if
a platform disagrees / comes up with new sane configurations.  This is
certainly the most difficult area to lock down because we have no idea
who is going to `innovate` and how. 

~Gregory