linux-kernel - Re: [PATCH v4 0/7] Add managed SOFT RESERVE resource handling

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Message-ID: <a1735579-82ef-4af7-b52d-52fe2d35483c@fujitsu.com>
Date: Wed, 4 Jun 2025 08:43:08 +0000
From: "Zhijian Li (Fujitsu)" <lizhijian@...itsu.com>
To: Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>,
	"linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
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>, Matthew Wilcox <willy@...radead.org>, Jan Kara
	<jack@...e.cz>, "Rafael J . Wysocki" <rafael@...nel.org>, Len Brown
	<len.brown@...el.com>, Pavel Machek <pavel@...nel.org>, Li Ming
	<ming.li@...omail.com>, Jeff Johnson <jeff.johnson@....qualcomm.com>, Ying
 Huang <huang.ying.caritas@...il.com>, "Xingtao Yao (Fujitsu)"
	<yaoxt.fnst@...itsu.com>, Peter Zijlstra <peterz@...radead.org>, Greg KH
	<gregkh@...uxfoundation.org>, Nathan Fontenot <nathan.fontenot@....com>,
	Terry Bowman <terry.bowman@....com>, Robert Richter <rrichter@....com>,
	Benjamin Cheatham <benjamin.cheatham@....com>, PradeepVineshReddy Kodamati
	<PradeepVineshReddy.Kodamati@....com>
Subject: Re: [PATCH v4 0/7] Add managed SOFT RESERVE resource handling

Smita,

Thanks for your awesome work. I just tested the scenarios you listed, and they work as expected. Thanks again.
(Minor comments inlined)

Tested-by: Li Zhijian <lizhijian@...itsu.com>


To the CXL community,

The scenarios mentioned here essentially cover what a correct firmware may provide. However,
I would like to discuss one more scenario that I can simulate with a modified QEMU:
The E820 exposes a SOFT RESERVED region which is the same as a CFMW, but the HDM decoders are not committed. This means no region will be auto-created during boot.

As an example, after boot, the iomem tree is as follows:
1050000000-304fffffff : CXL Window 0
   1050000000-304fffffff : Soft Reserved
     <No region>

In this case, the SOFT RESERVED resource is not trimmed, so the end-user cannot create a new region.
My question is: Is this scenario a problem? If it is, should we fix it in this patchset or create a new patch?




On 04/06/2025 06:19, Smita Koralahalli wrote:
> Add the ability to manage SOFT RESERVE iomem resources prior to them being
> added to the iomem resource tree. This allows drivers, such as CXL, to
> remove any pieces of the SOFT RESERVE resource that intersect with created
> CXL regions.
> 
> The current approach of leaving the SOFT RESERVE resources as is can cause
> failures during hotplug of devices, such as CXL, because the resource is
> not available for reuse after teardown of the device.
> 
> The approach is to add SOFT RESERVE resources to a separate tree during
> boot.

No special tree at all since V3


> This allows any drivers to update the SOFT RESERVE resources before
> they are merged into the iomem resource tree. In addition a notifier chain
> is added so that drivers can be notified when these SOFT RESERVE resources
> are added to the ioeme resource tree.
> 
> The CXL driver is modified to use a worker thread that waits for the CXL
> PCI and CXL mem drivers to be loaded and for their probe routine to
> complete. Then the driver walks through any created CXL regions to trim any
> intersections with SOFT RESERVE resources in the iomem tree.
> 
> The dax driver uses the new soft reserve notifier chain so it can consume
> any remaining SOFT RESERVES once they're added to the iomem tree.
> 
> The following scenarios have been tested:
> 
> Example 1: Exact alignment, soft reserved is a child of the region
> 
> |---------- "Soft Reserved" -----------|
> |-------------- "Region #" ------------|
> 
> Before:
>    1050000000-304fffffff : CXL Window 0
>      1050000000-304fffffff : region0
>        1050000000-304fffffff : Soft Reserved
>          1080000000-2fffffffff : dax0.0

BTW, I'm curious how to set up a dax with an address range different from its corresponding region.


>            1080000000-2fffffffff : System RAM (kmem)
> 
> After:
>    1050000000-304fffffff : CXL Window 0
>      1050000000-304fffffff : region1
>        1080000000-2fffffffff : dax0.0
>          1080000000-2fffffffff : System RAM (kmem)
> 
> Example 2: Start and/or end aligned and soft reserved spans multiple
> regions

Tested

> 
> |----------- "Soft Reserved" -----------|
> |-------- "Region #" -------|
> or
> |----------- "Soft Reserved" -----------|
> |-------- "Region #" -------|

Typo? should be:
|----------- "Soft Reserved" -----------|
             |-------- "Region #" -------|

> 
> Example 3: No alignment
> |---------- "Soft Reserved" ----------|
> 	|---- "Region #" ----|

Tested.


Thanks
Zhijian