linux-kernel - Re: [PATCH 0/4 v4] cxl/core: Enable Region creation/attach on x86 with LMH

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Message-ID: <91f42134-851d-446b-8b60-f93c429cfc59@intel.com>
Date: Thu, 4 Sep 2025 17:07:36 -0700
From: Dave Jiang <dave.jiang@...el.com>
To: "Fabio M. De Francesco" <fabio.m.de.francesco@...ux.intel.com>,
 linux-cxl@...r.kernel.org
Cc: Davidlohr Bueso <dave@...olabs.net>,
 Jonathan Cameron <jonathan.cameron@...wei.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
Subject: Re: [PATCH 0/4 v4] cxl/core: Enable Region creation/attach on x86
 with LMH



On 7/24/25 7:20 AM, 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. 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, the driver fails and returns an error because it
> expects that the CXL Endpoint Decoder range is a subset of the Root
> Decoder's (SPA >= HPA). On x86 with LMH's, it happens that SPA < HPA.
> 
> Therefore, detect x86 Low Memory Holes, match CXL Root and Endpoint
> Decoders or already made CXL Regions and Decoders to allow the
> construction of new CXL Regions and the attachment of Endpoint Decoders,
> even if SPA < HPA. If needed because of LMH's, adjust the Endpoint Decoder
> range end to match Root Decoder's.
> 
> - Patch 1/4 changes the calling conventions of three match_*_by_range()
>   helpers in preparation of 3/4.
> - Patch 2/4 Introduces helpers to detect LMH's and also one to adjust
>   the HPA range end for CXL Regions construction.
> - Patch 3/4 enables CXL Regions construction and Endpoint Decoders
>   attachment by matching Root Decoders or Regions with Endpoint
>   Decoders, adjusting Endpoint Decoders HPA range end, and relaxing
>   constraints while Endpoints decoders' attachment.
> - Patch 4/4 simulates a LMH for the CXL tests on patched CXL driver.
> 
> Many thanks to Alison, Dan, Dave and Ira for their help.
> 
> Commenting on v1, Alison wrote a couple of observations on what users
> will see. I suggest anyone interested to see how this series affect
> users to take a look at her observations.[0] Thank you!
> 
> Changes for v4:
> 
>   Re-base on top of 
> 	"cxl: Address translation support, part 1: Cleanups and refactoring";[1] 
>   Drop no more necessary 2/4;
>   Drop collected tags because of major changes throughout the series.
> 
>   1/3 - Adjust Endpoint Decoders dpa_res->end (Alison) [2] 
>   3/3 - Use weak/strong mechanism (Dan) [3]
> 
> Changes for v3:
> 
>   Re-base the series on cxl/next.
> 
>   1/4 - 2/4:
>         Constify local variables.
>   3/4:
>         Call arch_match_region() from region_res_match_cxl_range().
>   4/4:
>         arch_match_region() - Check that region end is under start + 4G;
>         arch_match_spa() - Check that SPA range start is cfmws_range_start.
> 
> v3 - https://lore.kernel.org/linux-cxl/20250314113708.759808-1-fabio.m.de.francesco@linux.intel.com/
> v2 - https://lore.kernel.org/linux-cxl/20250114203432.31861-1-fabio.m.de.francesco@linux.intel.com/
> v1 - https://lore.kernel.org/all/20241122155226.2068287-1-fabio.m.de.francesco@linux.intel.com/
> 
> [0] - https://lore.kernel.org/all/Z0Tzif55CcHuujJ-@aschofie-mobl2.lan/
> [1] - https://lore.kernel.org/linux-cxl/20250509150700.2817697-1-rrichter@amd.com/
> [2] - https://lore.kernel.org/linux-cxl/Z9tzZkn1rqd2Uk_6@aschofie-mobl2.lan/
> [3] - https://lore.kernel.org/linux-cxl/67ee07cd4f8ec_1c2c6294d5@dwillia2-xfh.jf.intel.com.notmuch/
> 
> Fabio M. De Francesco (4):
>   cxl/core: Change match_*_by_range() signatures
>   cxl/core: Add helpers to detect Low Memory Holes on x86
>   cxl/core: Enable Region creation on x86 with LMH
>   cxl/test: Simulate an x86 Low Memory Hole for tests
> 
>  drivers/cxl/Kconfig               |   5 ++
>  drivers/cxl/core/Makefile         |   1 +
>  drivers/cxl/core/platform.c       |  86 +++++++++++++++++++
>  drivers/cxl/core/platform.h       |  32 +++++++
>  drivers/cxl/core/region.c         | 113 ++++++++++++++++--------
>  tools/testing/cxl/Kbuild          |   2 +
>  tools/testing/cxl/mock_platform.c | 137 ++++++++++++++++++++++++++++++
>  tools/testing/cxl/test/cxl.c      |  10 +++
>  tools/testing/cxl/test/mock.h     |   1 +
>  9 files changed, 352 insertions(+), 35 deletions(-)
>  create mode 100644 drivers/cxl/core/platform.c
>  create mode 100644 drivers/cxl/core/platform.h
>  create mode 100644 tools/testing/cxl/mock_platform.c
> 
> base-commit: acc2913692413df9d1

The series doesn't seem to apply to this base commit. 
>