linux-kernel - [LSFMM] Updated: Linux Management of Volatile CXL Memory Devices

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Message-ID: <Z9mo5qCpdlE-KZ7P@gourry-fedora-PF4VCD3F>
Date: Tue, 18 Mar 2025 13:09:58 -0400
From: Gregory Price <gourry@...rry.net>
To: lsf-pc@...ts.linux-foundation.org
Cc: linux-mm@...ck.org, linux-cxl@...r.kernel.org,
	linux-kernel@...r.kernel.org, mhocko@...e.com
Subject: [LSFMM] Updated: Linux Management of Volatile CXL Memory Devices

Updating the summary to this because the old one no longer describes the
goal of the session well enough.

---

I'd like to propose a discussion of how CXL volatile memory device
configuration is creating a complex memory initialization/allocation
process.  This is making it difficult for users to get "what they want".

Example issues for discussion:
- Linux-arch defined memory block size and alignment requirements
- CXL as ZONE_NORMAL vs ZONE_MOVABLE impacts on memmap and performance
- NUMA node and memory tier allocations, and new evolutions
  a) multiple NUMA-nodes per device
  b) confusing state of memory-tier component
- cma and 1GB hugepage allocation from ZONE_MOVABLE on CXL

Most of this complexity is being generated by how BIOS, ACPI, build, and
boot config options interactions with each other. The best documented
description of these interactions is: "Complicated, Subtle, and Scary".

Having better documentation on "Linux Expectations for CXL Platforms"
will help guide innovations in this space - but requires some agreement
on limitations to keep existing components functional. This is becoming
increasingly important as more CXL hardware is placed into production.

The live session will not cover the full initialization process, which
I've attempted to describe and discuss in [1][2][3][4][5][6][7][8].

The intent is to evolve this series into more explicit documentation for
platform developers and help bridge the gap between maintainers that
need background on "Why the heck does CXL need this?".

[1] Section 0: ACPI Tables
https://lore.kernel.org/linux-mm/Z8jORKIWC3ZwtzI4@gourry-fedora-PF4VCD3F/

[2] Section 0a: CFMWS and NUMA Flexibility
https://lore.kernel.org/linux-mm/Z8u4GTrr-UytqXCB@gourry-fedora-PF4VCD3F/

[3] Section 1: BIOS, EFI, and Early Boot
https://lore.kernel.org/linux-mm/Z6LKJZkcdjuit2Ck@gourry-fedora-PF4VCD3F/

[4] Section 2: The Drivers
https://lore.kernel.org/linux-mm/Z6OMcLt3SrsZjgvw@gourry-fedora-PF4VCD3F/

[5] Section 2a: CXL Decoder Programming
https://lore.kernel.org/linux-mm/Z8o2HfVd0P_tMhV2@gourry-fedora-PF4VCD3F/

[6] Section 3: Memory (block) Hotplug
https://lore.kernel.org/linux-mm/Z7OWmDXEYhT0BB0X@gourry-fedora-PF4VCD3F/

[7] Section 4: Interleave
https://lore.kernel.org/linux-mm/Z9DQnjPWbkjqrI9n@gourry-fedora-PF4VCD3F/

[8] Section 5: Page Allocation
https://lore.kernel.org/linux-mm/Z9Ogp9fCEPoORfnh@gourry-fedora-PF4VCD3F/

~Gregory