linux-kernel - Re: [PATCH 2/4 v2] cxl/core: Add helpers to detect Low memory Holes on x86

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Message-ID: <Z4ccKD9Fc-Egc6EL@gourry-fedora-PF4VCD3F>
Date: Tue, 14 Jan 2025 21:23:36 -0500
From: Gregory Price <gourry@...rry.net>
To: "Fabio M. De Francesco" <fabio.m.de.francesco@...ux.intel.com>
Cc: Davidlohr Bueso <dave@...olabs.net>,
	Jonathan Cameron <jonathan.cameron@...wei.com>,
	Dave Jiang <dave.jiang@...el.com>,
	Alison Schofield <alison.schofield@...el.com>,
	Vishal Verma <vishal.l.verma@...el.com>,
	Ira Weiny <ira.weiny@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Robert Richter <rrichter@....com>, ming.li@...omail.com,
	linux-kernel@...r.kernel.org, linux-cxl@...r.kernel.org
Subject: Re: [PATCH 2/4 v2] cxl/core: Add helpers to detect Low memory Holes
 on x86

On Tue, Jan 14, 2025 at 09:32:54PM +0100, Fabio M. De Francesco wrote:
> +/*
> + * Match CXL Root and Endpoint Decoders by comparing SPA and HPA ranges.
> + *
> + * On x86, CFMWS ranges never intersect memory holes while endpoint decoders
> + * HPA range sizes are always guaranteed aligned to NIW * 256MB; therefore,
> + * the given endpoint decoder HPA range size is always expected aligned and
> + * also larger than that of the matching root decoder. If there are LMH's,
> + * the root decoder range end is always less than SZ_4G.
> + */

Is there any reason to limit this memory-hole handling to only low
memory holes? I have observed systems where the following memory
hole situation occurs:

(example, not exact sizes)
CFMW1:   [  0xc0000000 - 0xdfffffff  ] 512MB range
Reserved [  0xe0000000 - 0xffffffff  ] 512MB range
CFMW2:   [ 0x100000000 - 0x15fffffff ] 1.5GB range

2 CXL Memory Devices w/ 1GB capacity each (again, an example).

Note that 1 device has its capacity split across the hole, but
if the devices are interleaved then both devices have their capacity
split across the hole.

It seems with some mild modification, this patch set could be
re-used to handle this memory hole scenario as well
(w/ addr translation - Robert's patch set)

Is there a reason not to handle more than just LMH's in this set?

(I may try to hack this up on my test system and report back.)

~Gregory