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Message-ID: <2dd4d4fe-3680-456b-812b-8e3e99f2cb8b@fujitsu.com>
Date: Mon, 1 Sep 2025 02:46:32 +0000
From: "Zhijian Li (Fujitsu)" <lizhijian@...itsu.com>
To: "Koralahalli Channabasappa, Smita"
<Smita.KoralahalliChannabasappa@....com>, Alison Schofield
<alison.schofield@...el.com>
CC: "dan.j.williams@...el.com" <dan.j.williams@...el.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>, 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>, 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>, "Yasunori Gotou (Fujitsu)"
<y-goto@...itsu.com>
Subject: Re: [PATCH v5 3/7] cxl/acpi: Add background worker to coordinate with
cxl_mem probe completion
On 29/08/2025 07:21, Koralahalli Channabasappa, Smita wrote:
> Hi Zhijian,
>
> On 8/26/2025 11:30 PM, Zhijian Li (Fujitsu) wrote:
>> All,
>>
>>
>> I have confirmed that in the !CXL_REGION configuration, the same environment may fail to fall back to hmem.(Your new patch cannot resolve this issue)
>>
>> In my environment:
>> - There are two CXL memory devices corresponding to:
>> ```
>> 5d0000000-6cffffff : CXL Window 0
>> 6d0000000-7cffffff : CXL Window 1
>> ```
>> - E820 table contains a 'soft reserved' entry:
>> ```
>> [ 0.000000] BIOS-e820: [mem 0x00000005d0000000-0x00000007cfffffff] soft reserved
>> ```
>>
>> However, since my ACPI SRAT doesn't describe the CXL memory devices (the point), `acpi/hmat.c` won't allocate memory targets for them. This prevents the call chain:
>> ```c
>> hmat_register_target_devices() // for each SRAT-described target
>> -> hmem_register_resource()
>> -> insert entry into "HMEM devices" resource
>> ```
>>
>> Therefore, for successful fallback to hmem in this environment: `dax_hmem.ko` and `kmem.ko` must request resources BEFORE `cxl_acpi.ko` inserts 'CXL Window X'
>>
>> However the kernel cannot guarantee this initialization order.
>>
>> When cxl_acpi runs before dax_kmem/kmem:
>> ```
>> (built-in) CXL_REGION=n
>> driver/dax/hmem/device.c cxl_acpi.ko dax_hmem.ko kmem.ko
>>
>> (1) Add entry '15d0000000-7cfffffff'
>> (2) Traverse "HMEM devices"
>> Insert to iomem:
>> 5d0000000-7cffffff : Soft Reserved
>>
>> (3) Insert CXL Window 0/1
>> /proc/iomem shows:
>> 5d0000000-7cffffff : Soft Reserved
>> 5d0000000-6cffffff : CXL Window 0
>> 6d0000000-7cffffff : CXL Window 1
>>
>> (4) Create dax device
>> (5) request_mem_region() fails
>> for 5d0000000-7cffffff
>> Reason: Children of 'Soft Reserved'
>> (CXL Windows 0/1) don't cover full range
>> ```
>>
>
> Thanks for confirming the failure point. I was thinking of two possible ways forward here, and I would like to get feedback from others:
>
> [1] Teach dax_hmem to split when the parent claim fails:
> If __request_region() fails for the top-level Soft Reserved range because IORES_DESC_CXL children already exist, dax_hmem could iterate those windows and register each one individually. The downside is that it adds some complexity and feels a bit like papering over the fact that CXL should eventually own all of this memory.
I examined below change to ensure kmem runs first, it seemed to work.
static int __init cxl_acpi_init(void)
{
+ if (!IS_ENABLED(CONFIG_DEV_DAX_CXL) && IS_ENABLED(CONFIG_DEV_DAX_KMEM)) {
+ /* fall back to dax_hmem,kmem */
+ request_module("kmem");
+ }
return platform_driver_register(&cxl_acpi_driver);
}
> As Dan mentioned, the long-term plan is for Linux to not need the soft-reserve fallback at all, and simply ignore Soft Reserve for CXL Windows because the CXL subsystem will handle it.
The current CXL_REGION kconfig states:
Otherwise, platform-firmware managed CXL is enabled by being placed in the system address map and does not need a driver.
I think this implies that a fallback to dax_hmem/kmem is still required for such cases.
Of course, I personally agree with this 'long-term plan'.
>
> [2] Always unconditionally load CXL early..
> Call request_module("cxl_acpi"); request_module("cxl_pci"); from dax_hmem_init() (without the IS_ENABLED(CONFIG_DEV_DAX_CXL) guard). If those are y/m, they’ll be present; if n, it’s a no-op. Then in hmem_register_device() drop the IS_ENABLED(CONFIG_DEV_DAX_CXL) gate and do:
>
> if (region_intersects(res->start, resource_size(res),
> IORESOURCE_MEM, IORES_DESC_CXL) !=REGION_DISJOINT)
> /* defer to CXL */;
>
> and defer to CXL if windows are present. This makes Soft Reserved unavailable once CXL Windows have been discovered, even if CXL_REGION is disabled. That aligns better with the idea that “CXL should win” whenever a window is visible (This also needs to be considered alongside patch 6/6 in my series.)
>
> With CXL_REGION=n there would be no devdax and no kmem for that range; proc/iomem would show only the windows something like below
>
> 850000000-284fffffff : CXL Window 0
> 2850000000-484fffffff : CXL Window 1
> 4850000000-684fffffff : CXL Window 2
>
> That means the memory is left unclaimed/unavailable.. (no System RAM, no /dev/dax). Is that acceptable when CXL_REGION is disabled?
Regarding option [2] (unconditionally loading CXL early):
This approach conflicts with the CXL_REGION Kconfig description mentioned above.
---
To refocus on the original issue – the inability to recreate regions after destruction when CXL Windows overlap with Soft Reserved
I believe your patch series "[PATCH 0/6] dax/hmem, cxl: Coordinate Soft Reserved handling with CXL" effectively addresses this problem.
As for the pre-existing issues with !CXL_REGION and the unimplemented DAX_CXL_MODE_REGISTER, I suggest deferring them for now.
They need not be resolved within this patch set, as we should prioritize the initial problem.
Thanks
Zhijian
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