linux-kernel - Re: [PATCH 2/4 v3] cxl/core: Add helpers to detect Low memory Holes on x86

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Message-ID: <67d98e0394e3a_422c1294e8@iweiny-mobl.notmuch>
Date: Tue, 18 Mar 2025 10:15:15 -0500
From: Ira Weiny <ira.weiny@...el.com>
To: "Fabio M. De Francesco" <fabio.m.de.francesco@...ux.intel.com>, "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>
CC: Robert Richter <rrichter@....com>, <ming.li@...omail.com>,
	<linux-kernel@...r.kernel.org>, <linux-cxl@...r.kernel.org>, "Fabio M. De
 Francesco" <fabio.m.de.francesco@...ux.intel.com>
Subject: Re: [PATCH 2/4 v3] cxl/core: Add helpers to detect Low memory Holes
 on x86

Fabio M. De Francesco wrote:
> In x86 with Low memory Hole, the BIOS may publishes CFMWS that describe
> SPA ranges which are subsets of the corresponding CXL Endpoint Decoders
> HPA's because the CFMWS never intersects LMH's while EP Decoders HPA's
> ranges are always guaranteed to align to the NIW * 256M rule.
> 
> In order to construct Regions and attach Decoders, the driver needs to
> match Root Decoders and Regions with Endpoint Decoders, but it fails and
> the entire process returns errors because it doesn't expect to deal with
> SPA range lengths smaller than corresponding HPA's.
> 
> Introduce functions that indirectly detect x86 LMH's by comparing SPA's
> with corresponding HPA's. They will be used in the process of Regions
> creation and Endpoint attachments to prevent driver failures in a few
> steps of the above-mentioned process.
> 
> The helpers return true when HPA/SPA misalignments are detected under
> specific conditions: both the SPA and HPA ranges must start at
> LMH_CFMWS_RANGE_START (that in x86 with LMH's is 0x0), SPA range sizes
> be less than HPA's, SPA's range's size be less than 4G, HPA's size be
> aligned to the NIW * 256M rule.
> 
> Also introduce a function to adjust the range end of the Regions to be
> created on x86 with LMH's.
> 
> Cc: Alison Schofield <alison.schofield@...el.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Ira Weiny <ira.weiny@...el.com>

Reviewed-by: Ira Weiny <ira.weiny@...el.com>

[snip]