Re: [RFC PATCH 0/3] cxl: Auto-create a region for Type-2 memdev attach

[Richard Cheng]

On Tue, Aug 25, 2026 at 10:50:06AM +0800, Lucero Palau, Alejandro wrote:
> Hi Richard,
> 
> On 20/08/2026 10:41, Richard Cheng wrote:
> > On Wed, Aug 12, 2026 at 10:58:02AM +0800, Alejandro Lucero Palau wrote:
> > > Hi Richard,
> > > 
> > > 
> > > Some comments below.
> > > 
> > > 
> > > Thanks!
> > > 
> > > 
> > > On 8/5/26 08:40, Richard Cheng wrote:
> 
> 
> <snip>
> 
> > Hi Alejandro,
> > 
> > Thanks for the review and explanation. I've read them all.
> > 
> > I think you are right that this RFC doesn't currently have a production platform
> > where the system FW publishes a Type-2 CFMWS but leaves the EP decoder
> > uncommitted.
> > 
> > However, the config appears to be permitted by the CXL model. A CFMWS describes
> > a FW-established root HPA window and the restrictions governing its use,
> > including Type-2 v.s. Type-3 and volatile v.s. PMEM. The CFMWS def also
> > describes OSPM assigning HPA ranges from those windows to discovered CXL.mem
> > devices [1].
> 
> 
> Right. I'm not saying this should not be supported, just pointing out the
> use case does not make sense with current BIOS functionality. I think BIOS
> will/could support a config option for just leaving a Type2 HDM uncommitted,
> but then why the kernel should do the same a default BIOS config would do?
>

Agreed. The kernel shouldn't recreate the config that BIOS would normally provide.
And that's why I think we should move region createion out of devm_cxl_probe_mem().
That helper should discover and attach to an already committed region.

If FW leaves the decoders unconfigured intentionally , a driver may explicitly request a region
and provide the size it needs.

In my mind the new model should be
- FW-committed config is only discovered and attached
- an uncommitted config remains untouched unless a driver explicitly requests it
- CXL core supplieds the allocation, validation, programming, accounting and teardown mechnism
- the requesting driver owns the policy and the use of the region

So far I think PMEM reconstruction form label data maybe be a potential use case, though it's not
provided in kernel right now, we can work on it in the future, or work on that one first and we'll
continue the auto-create region part.

What do you think ?

 
> 
> > 
> > The Linux CXL doc similarly states that only root decoders are required to be
> > programmed during probe. Switch and EP decoder may remain available for runtime
> > programming when the platform supports it [2].
> 
> 
> Tangential to this discussion, but I have problems with this assertion. Any
> switch or EP HDM programming will need a root port HDM programming as well.
> Not sure which root decoders will need to be programmed at boot time: a
> CFMWS is "programmed" by the BIOS and root decoders will need to be
> programmed as well for any Type2/switch found with an enabled link.
> 
> 

Here root decoder I mean the logical object created from a CFMWS. I didn't mean switch or EP
decoder can be programmed independently.

My point was that FW establishes the CFMWS window, while the platform may leave downstream decoder
path for later programming.

> > 
> > I raise the RFC intended for the question of how Linux should support that
> > architecturally permitted config.
> > 
> > The cxl_test config added in patch 3/3 constructs this scenario synthetically.
> > This demonstrates the proposed kernel behavior, but I agree I don't know
> > whether there exists a deployed FW scenario.
> > 
> > 
> > And I agree that devm_cxl_probe_mem() shouldn't silently change from
> > "attach to a FW-established region" into "allocate resources and program a new
> > region". Those operations should have different semantics and ownership
> > expectations.
> 
> 
> Glad with the consensus :-)
> 
> 
> > 
> > I am planning to rebase onto cxl/nexxt and rework the proposal as the following,
> > please take a look and see if that matches your imagination or not.
> > * Keep devm_cxl_probe_mem() behavior unchanged for FW-committed regions
> > * Make region creation an explicit request from the accelerator provider,
> >    rather than an automatic fallback during memdev attach
> > * Have the provider specify the required size. CXL core shouldn't assume
> >    that it maybe consume the entire volatile DPA partition as you mentioned.
> > * Separate the reusable region-provisioning mechanism from the initial Type-2
> >    policy.
> > * The common mechanism should handle HPA/DPA allocation, decoder-path
> >    construction, commit , rollback and managed teardown.
> > * The initial type-2 caller would constrain that to volatile DEVMEM, IW=1
> >    and a provider-requested size.
> > 
> > Oh and I'll replace "x1" with "IW=1" and explain the initial decoder,
> > root-selection and granularity restriction more clearly.
> > 
> > How does that sound to you ?
> 
> 
> It sounds perfect!
> 
> 
> FWIW, you likely saw Gregory's comment (discord) on this work requiring the
> support for PMEM or at least the awareness PMEM support will need to use
> same interface. His opinion and mine came from Dan's vision on this, and
> your work will be the base for such PMEM support. I do not have an impending
> reason for working on this PMEM support, but I am really interested in how
> Type3 PMEMs can leverage CXL.mem for improving storage needs, and currently
> reading/thinking about all this ...
> 
> 
> Thanks!
> 

Hmmm for this part I have no idea for now, I'll study more and discuss with you guys.

Best regards,
Richard Cheng.

> 
> > [1]: https://computeexpresslink.org/wp-content/uploads/2024/02/CEDT_ECN_1.0A_Eval.pdf
> > [2]: https://docs.kernel.org/driver-api/cxl/linux/cxl-driver.html#runtime-programming
> > 
> > Best regards,
> > Richard Cheng.
> > > > Testing result is in the following.
> > > > - Built clean with clang/LLVM on arm64
> > > > - cxl_test, type2_test=1. accel0 takes the unchanged attach path. accel1
> > > >     drives auto_create -> a committed 512 MB RAM region. The test asserts
> > > >     the 512 MB HPA range. committed state and 256 byte granularity confirmed
> > > >     via sysfs.
> > > > - Unbind tears the region down with no orphaned decoder, rebind re-creates
> > > >     a fresh committed region.
> > > > - Mock test only. Real accelerators whose FW commits a decoder take the
> > > >     attach path, and vfio-cxl binds only FW-committed devices, so auto-create
> > > >     has no real-HW caller yet.
> > > > 
> > > > Best regards,
> > > > Richard Cheng.
> > > > 
> > > > Richard Cheng (3):
> > > >     cxl/region: Reset software-created regions on memdev detach
> > > >     cxl/region: Auto-create a region for memdev attach
> > > >     cxl/test: Exercise Type-2 automatic region creation
> > > > 
> > > >    drivers/cxl/core/region.c      | 422 +++++++++++++++++++++++++++++----
> > > >    tools/testing/cxl/test/accel.c |   7 +
> > > >    tools/testing/cxl/test/cxl.c   |  61 ++++-
> > > >    3 files changed, 439 insertions(+), 51 deletions(-)
> > > > 
> > > > 
> > > > base-commit: 1c6b4ceafc3b994871c29340e0c1ddb0af5800e7