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

Yasunori Goto (Fujitsu)
2026-02-16T05:16:01Z

> 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/OS9PR01MB12421AEA8B27BF942CD0F18B1
> 90
> > 57A@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.

I misunderstood it
I'll re-check the specifications.
Sorry for the noise.

>
> 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.

Thank you very much.

-----
Yasunori Goto