linux-kernel - Re: [PATCH 0/4 v3] cxl/core: Enable Region creation on x86 with Low Mem Hole

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Message-ID: <3287167.Hq7AAxBmiT@fdefranc-mobl3>
Date: Wed, 26 Mar 2025 17:24:59 +0100
From: "Fabio M. De Francesco" <fabio.m.de.francesco@...ux.intel.com>
To: Alison Schofield <alison.schofield@...el.com>
Cc: Davidlohr Bueso <dave@...olabs.net>,
 Jonathan Cameron <jonathan.cameron@...wei.com>,
 Dave Jiang <dave.jiang@...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, linux-cxl@...r.kernel.org
Subject:
 Re: [PATCH 0/4 v3] cxl/core: Enable Region creation on x86 with Low Mem Hole

On Thursday, March 20, 2025 7:10:13 PM Central European Standard Time Alison Schofield wrote:
> On Fri, Mar 14, 2025 at 12:36:29PM +0100, 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.
> 
> Unless dev_dbg() is on, LMH handling won't be apparent. How about adding
> a dev_info() that informs the user that a region was adjusted to account
> for a LMH. I'm foreseeing looking at logs and divining a LMH, when a
> simple log message would be a nice shortcut.
> 
> CXL subsystem is quiet with only 8 dev_info()'s, so if this breaks some
> vow of silence, nevermind :)
> 
Yes, sure. It sounds good.

Thanks,

Fabio

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