Re: [PATCH v3] cxl: docs/driver-api/conventions resolve conflicts btw CFMWS, LMH, ED

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Mon, 23 Jun 2025 17:29:02 +0200
"Fabio M. De Francesco" <fabio.m.de.francesco@linux.intel.com> wrote:

> Add documentation on how to resolve conflicts between CXL Fixed Memory
> Windows, Platform Memory Holes, and Endpoint Decoders.
> 
> Signed-off-by: Fabio M. De Francesco <fabio.m.de.francesco@linux.intel.com>
> ---
> 
> v2 -> v3: Rework a few phrases for better clarity.
> 	  Fix grammar and syntactic errors (Randy, Alok).
> 	  Fix semantic errors ("size does not comply", Alok).
> 	  Fix technical errors ("decoder's total memory?", Alok).
> 	  
> v1 -> v2: Rewrite "Summary of the Change" section, 3r paragraph.
> 
>  Documentation/driver-api/cxl/conventions.rst | 85 ++++++++++++++++++++
>  1 file changed, 85 insertions(+)
> 
> diff --git a/Documentation/driver-api/cxl/conventions.rst b/Documentation/driver-api/cxl/conventions.rst
> index da347a81a237..d6c8f4cf2f5b 100644
> --- a/Documentation/driver-api/cxl/conventions.rst
> +++ b/Documentation/driver-api/cxl/conventions.rst
> @@ -45,3 +45,88 @@ Detailed Description of the Change
>  ----------------------------------
>  
>  <Propose spec language that corrects the conflict.>
> +
> +
> +Resolve conflict between CFMWS, Platform Memory Holes, and Endpoint Decoders
> +============================================================================
> +
> +Document
> +--------
> +
> +CXL Revision 3.2, Version 1.0
> +
> +License
> +-------
> +
> +SPDX-License Identifier: CC-BY-4.0
> +
> +Creator/Contributors
> +--------------------
> +
> +Fabio M. De Francesco, Intel
> +Dan J. Williams, Intel
> +Mahesh Natu, Intel
> +
> +Summary of the Change
> +---------------------
> +
> +According to the current CXL Specifications (Revision 3.2, Version 1.0)
> +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 range that may be
> +interleaved across one or more targets, some of which are CXL Host Bridges.
> +Associated with each window is a set of restrictions that govern its usage.
> +It is the OSPM’s responsibility to utilize each window for the specified
> +use.
> +
> +Table 9-22 states the Window Size field contains the total number of
> +consecutive bytes of HPA this window represents and this value shall be a
> +multiple of Number of Interleave Ways * 256 MB.
> +
> +Platform Firmware (BIOS) might reserve part of physical addresses below
> +4 GB (e.g., the Low Memory Hole that describes PCIe memory space for MMIO
> +or a requirement for the greater than 8 way interleave CXL regions starting
> +at address 0). In that case the Window Size value cannot be anymore
> +constrained to the NIW * 256 MB above-mentioned rule.

I'm not following argument for large interleave at address 0 being a problem
(if we ignore the low memory hole and similar as a separate issue).  Even
if it is the interaction with the low memory hole, is 12 way interleave
of 256MiB devices a problem?  Fills up to 3GiB.


> +
> +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.
> +
> +The description of the Window Size field in table 9-22 needs to take that
> +special case into account.
> +
> +Note that the Endpoint Decoders HPA range sizes have to comply with the
> +alignment constraints and so a part of their memory capacity might not be
> +accessible if their size exceeds the matching CFMWS range's.
> +
> +Benefits of the Change
> +----------------------
> +
> +Without this change, the OSPM wouldn't match Endpoint Decoders with CFMWS
> +whose Window Size don't fit the alignment constraints and so the memdev
> +capacity would be lost. This change allows the OSPM to match Endpoint
> +Decoders whose HPA range size exceeds the matching CFMWS and create
> +regions that at least utilize a part of the memory devices total capacity.
> +
> +References
> +----------
> +
> +Compute Express Link Specification Revision 3.2, Version 1.0
> +<https://www.computeexpresslink.org/>
> +
> +Detailed Description of the Change
> +----------------------------------
> +
> +The current description of a CFMWS Window Size (Table 9-22) is replaced
> +with:
> +
> +"The total number of consecutive bytes of HPA this window represents. This
> +value shall be a multiple of NIW*256 MB. On platforms that reserve physical
> +addresses below 4 GB for special use (e.g., the Low Memory Hole for PCIe
> +MMIO on x86), an instance of CFMWS whose Base HPA is 0 might have a window
> +size that doesn't align with the NIW*256 MB constraint; note that the
> +matching Endpoint Decoders HPA range size must still align to the
> +above-mentioned rule and so the memory capacity that might exceed the CFMWS
> +window size will not be accessible.".
> 
> base-commit: a021802c18c4c30dff3db9bd355cacb68521f1aa