linux-kernel - Re: [PATCH 3/4 v3] cxl/core: Enable Region creation on x86 with Low Memory Hole

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Message-ID: <67d9d9089eb52_422c129445@iweiny-mobl.notmuch>
Date: Tue, 18 Mar 2025 15:35:20 -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 3/4 v3] cxl/core: Enable Region creation on x86 with Low
 Memory Hole

Fabio M. De Francesco wrote:
> The CXL Fixed Memory Window Structure (CFMWS) describes zero or more Host
> Physical Address (HPA) windows that are associated with each CXL Host
> Bridge. Each window represents a contiguous HPA that may be interleaved
> with one or more targets (CXL v3.1 - 9.18.1.3).
> 
> The Low Memory Hole (LMH) of x86 is a range of addresses of physical low
> memory to which systems cannot send transactions. In some cases the size
> of that hole is not compatible with the CXL hardware decoder constraint
> that the size is always aligned to 256M * Interleave Ways.
> 
> On those systems, BIOS publishes CFMWS which communicate the active System
> Physical Address (SPA) ranges that map to a subset of the Host Physical
> Address (HPA) ranges. The SPA range trims out the hole, and capacity in
> the endpoint is lost with no SPA to map to CXL HPA in that hole.
> 
> In the early stages of CXL Regions construction and attach on platforms
> with Low Memory Holes, cxl_add_to_region() fails and returns an error
> because it can't find any CXL Window that matches a given CXL Endpoint
> Decoder.
> 
> Detect a Low Memory Hole by comparing Root Decoders and Endpoint Decoders
> ranges with the use of arch_match_{spa,region}() helpers.
> 
> Match Root Decoders and CXL Regions with corresponding CXL Endpoint
> Decoders. Currently a Low Memory Holes would prevent the matching functions
> to return true.
> 
> Construct CXL Regions with HPA range's end adjusted to the matching SPA.
> 
> Allow the attach target process to complete by allowing Regions to not
> comply with alignment constraints (i.e., alignment to NIW * 256M rule).
> 
> 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]