Re: [PATCH v6 0/9] dax/hmem, cxl: Coordinate Soft Reserved handling with CXL and HMEM

Alison Schofield
2026-02-13T17:31:26Z

On Fri, Feb 13, 2026 at 07:47:08AM +0000, Yasunori Goto (Fujitsu) wrote:
> Hello, Alison-san,
>
> I would like to clarify your answer a bit more.
>
> > On Thu, Feb 12, 2026 at 03:44:15PM +0100, Tomasz Wolski wrote:
> > > >
> > > >FYI - I am able to confirm the dax regions are back for
> > > >no-soft-reserved case, and my basic hotplug flow works with v6.
> > > >
> > > >-- Alison
> > >
> > > Hello Alison,
> > >
> > > I wanted to ask about this scenario.
> > > Is my understanding correct that this fix is needed for cases without Soft
> > Reserve and:
> > > 1) CXL memory is installed in the server (no hotplug) and OS is
> > > started
> > > 2) CXL memory is hot-plugged after the OS starts
> > > 3) Tests with cxl-test driver
> > or QEMU
>
> Though I can understand that cases 2) and 3) include QEMU, I'm not sure why Linux drivers must handle case 1).
> In such a case, I feel that the platform vendor should modify the firmware to define EFI_MEMORY_SP.
>
> In the past, I actually encountered another issue between our platform firmware and a Linux driver:
> https://lore.kernel.org/linux-cxl/OS9PR01MB12421AEA8B27BF942CD0F18B19057A@OS9PR01MB12421.jpnprd01.prod.outlook.com/
> In that case, I asked our firmware team to modify the firmware, and the issue was resolved.
>
> Therefore, I would like to confirm why case 1) must be handled.
> Have any actual machines already been released with such firmware?
> Otherwise, is this just to prepare for a platform whose firmware cannot be fixed on the firmware side?

Maybe I'm misunderstanding Tomasz's Case 1), because this is not
a work-around for a firmware issue.

The CXL driver always tries to create DAX regions out of RAM regions.
That happens if the CXL region is BIOS defined 'auto' region or a
region requested via userspace. That is irregardless of Soft Reserved
existence. Soft-Reserved is not a requirement for CXL or DAX region
creation.

That piece broke in an earlier rev of this patchset [1] where the calls
to devm_cxl_add_dax_region(cxlr) started returning EPROBE_DEFER.

I intended to point out to Smita, that behavior is restored in v6.

--Alison

[1] https://lore.kernel.org/linux-cxl/aXMWzC8zf3bqIHJ0@aschofie-mobl2.lan/

>
> Thanks,
> ---
> Yasunori Goto
>
>