Re: [PATCH 2/4 v5] cxl/core: Add helpers to detect Low Memory Holes on x86

[Gregory Price]

On Mon, Oct 06, 2025 at 05:58:05PM +0200, Fabio M. De Francesco wrote:
> On a x86 platform with a low memory hole (LHM), the BIOS may publish
                                        ^^^ LMH ^^^

> CFMWS that describes a system physical address (SPA) range that
> typically is only a subset of the corresponding CXL intermediate switch
> and endpoint decoder's host physical address (HPA) ranges. The CFMWS
> range never intersects the LHM and so the driver instantiates a root
> decoder whose HPA range size doesn't fully contain the matching switch
> and endpoint decoders' HPA ranges.[1]
> 
> To construct regions and attach decoders, the driver needs to match root
> decoders and regions with endpoint decoders. The process fails and
> returns errors because the driver is not designed to deal with SPA
> ranges which are smaller than the corresponding hardware decoders HPA
> ranges.
> 
> Introduce two functions that indirectly detect the presence of x86 LMH
> and allow the matching between a root decoder or an already constructed
> region with a corresponding intermediate switch or endpoint decoder to
> enable the construction of a region and the subsequent attachment of the
> same decoders to that region.
> 
> These functions return true when SPA/HPA misalignments due to LMH's are
> detected under specific conditions:
> 
> - Both the SPA and HPA ranges must start at LMH_CFMWS_RANGE_START (i.e.,
>   0x0 on x86 with LMH's).
> - The SPA range's size is less than HPA's.
> - The SPA range's size is less than 4G.
> - The HPA range's size is aligned to the NIW * 256M rule.
> 
> Also introduce a function that adjusts the range end of a region to be
> constructed and the DPA range's end of the endpoint decoders that will
> be later attached to that region.
> 
> [1] commit 7a81173f3 ("cxl: Documentation/driver-api/cxl: Describe the x86 Low Memory Hole solution")
> 
> Cc: Alison Schofield <alison.schofield@xxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
> Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
> Signed-off-by: Fabio M. De Francesco <fabio.m.de.francesco@xxxxxxxxxxxxxxx>

lgmt

Reviewed-by: Gregory Price <gourry@xxxxxxxxxx>