linux-kernel - Re: [PATCH 4/4 v3] cxl/test: Simulate an x86 Low Memory Hole for tests

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Message-ID: <Z-Z_U7rBJFKeB33A@rric.localdomain>
Date: Fri, 28 Mar 2025 11:52:03 +0100
From: Robert Richter <rrichter@....com>
To: "Fabio M. De Francesco" <fabio.m.de.francesco@...ux.intel.com>
Cc: 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>, ming.li@...omail.com,
	linux-kernel@...r.kernel.org, linux-cxl@...r.kernel.org
Subject: Re: [PATCH 4/4 v3] cxl/test: Simulate an x86 Low Memory Hole for
 tests

On 26.03.25 17:58:50, Fabio M. De Francesco wrote:
> On Friday, March 21, 2025 11:42:09 AM Central European Standard Time Robert Richter wrote:
> > On 14.03.25 12:36:33, Fabio M. De Francesco wrote:
> > > Simulate an x86 Low Memory Hole for the CXL tests by changing the first
> > > mock CFMWS range size to 768MB and the CXL Endpoint Decoder HPA range sizes
> > > to 1GB.
> > > 
> > > Since the auto-created region of cxl-test uses mock_cfmws[0], whose range
> > > base address is typically different from the one published by the BIOS on
> > > real hardware, the driver would fail to create and attach CXL Regions if
> > > it was run on the mock environment created by cxl-tests.
> > > 
> > > Therefore, save the mock_cfmsw[0] range base_hpa and reuse it to match CXL
> > > Root Decoders and Regions with Endpoint Decoders when the driver is run on
> > > mock devices.
> > > 
> > > Since the auto-created region of cxl-test uses mock_cfmws[0], the
> > > LMH path in the CXL Driver will be exercised every time the cxl-test
> > > module is loaded. Executing unit test: cxl-topology.sh, confirms the
> > > region created successfully with a LMH.
> > > 
> > > Cc: Alison Schofield <alison.schofield@...el.com>
> > > Cc: Dan Williams <dan.j.williams@...el.com>
> > > Cc: Ira Weiny <ira.weiny@...el.com>
> > > Signed-off-by: Fabio M. De Francesco <fabio.m.de.francesco@...ux.intel.com>
> > > ---
> > >  drivers/cxl/core/lmh.c               | 35 ++++++++++++++++++++++++----
> > >  drivers/cxl/core/lmh.h               |  2 ++
> > 
> > Can you take a look to move all those changes to testing/? This
> > indicates the interface of your mock functions need improvement.
> > 
> > -Robert
> > 
> > >  tools/testing/cxl/cxl_core_exports.c |  2 ++
> > >  tools/testing/cxl/test/cxl.c         | 10 ++++++++
> > >  4 files changed, 45 insertions(+), 4 deletions(-)
> > 
> Your comment is not very detailed but I think you are suggesting to 
> override the lmh functions in testing. If that is the case, I don't think 
> that any function which is exported from and also called by cxl-core 
> can be override by testing with the strong/weak mechanism. 

I think you need to modify cxl_test to create a LMH hole (register a
mock bridge and root port with the necessary parameters, etc.) and
then run cxl_core unmodified.

-Robert

> 
> But I'm not an expert of linker related topics and not even sure that I 
> understood what you suggested.
> 
> Would you please elaborate more?
> 
> Thanks,
> 
> Fabio