[RFC PATCH v3 00/16] CXL 2.0 Support

[Ben Widawsky]

Changes since v2 [1]

  - Rebased onto v5.11-rc2
  - fixes for sparse (Ben)
  - Add (and modify) IOCTL number to ioctl-number.rst. (Randy)
  - Change @enable to @flags with enable field. (Dan)
  - Move command "name" to UAPI enum. (Dan)
  - Make command array use designated initializer. (Dan)
  - Avoid copies to/from payload registers (Ben)
  - Actually write the input payload length (Ben)
  - Return ENOTTY instead of EINVAL for missing Send command. (Dan)
  - Keep device locked and pinned during send commands. (Dan)
  - Validate input payload size doesn't exceed hardware's limit. (Ben)
  - Get rid of TAINT flag in UAPI. (Dan)
  - Spelling fixes. (Dan)
  - Document things that must be zero. (Dan)
  - Use new TAINT (Dan)
  - Replace cxl_debug with dev_debug (Dan)
    - Squash debug info into relevant patches (Ben)
  - Put CXL_CMD on a diet, #define the opcodes (Dan)
  - Switch mailbox sync from spinlock to mutex (Ben)
  - Store off (and debug print) hardware's payload size (Ben)
  - Fix length of GENMASK for mailbox payload (Ben)
  - Create concept of enabled commands. (Ben)
  - Use CEL to enable commands based on hardware. (Ben)
  - Move command struct definitions into mem.c (Ben)
  - Create concept of hidden commands, and kernel only commands (Ben)
  - Add payload min to sysfs (Ben)

---

The patches can be found here:
https://gitlab.com/bwidawsk/linux/-/commits/cxl-2.0v3
I've also created #cxl on oftc for any discussion.

---

Introduce support for “type-3” memory devices defined in the recently released
Compute Express Link (CXL) 2.0 specification[2]. Specifically, these are the
memory devices defined by section 8.2.8.5 of the CXL 2.0 spec. A reference
implementation emulating these devices has been submitted to the QEMU mailing
list [3] and is available on gitlab [4]. “Type-3” is a CXL device that acts as a
memory expander for RAM or PMEM. It might be interleaved with other CXL devices
in a given physical address range.

These changes allow for foundational enumeration of CXL 2.0 memory devices as
well as basic userspace interaction. The functionality present is:
- Initial driver bring-up
- Device enumeration and an initial sysfs representation
- Submit a basic firmware command via ‘mailbox’ to an emulated memory device
  with non-volatile capacity.
- Provide an interface to send commands to the hardware.

Some of the functionality that is still missing includes:
- Memory interleaving at the host bridge, root port, or switch level
- CXL 1.1 Root Complex Integrated Endpoint Support
- CXL 2.0 Hot plug support
- A bevy of supported device commands

In addition to the core functionality of discovering the spec defined registers
and resources, introduce a CXL device model that will be the foundation for
translating CXL capabilities into existing Linux infrastructure for Persistent
Memory and other memory devices. For now, this only includes support for the
management command mailbox that type-3 devices surface. These control devices
fill the role of “DIMMs” / nmemX memory-devices in LIBNVDIMM terms.

Now, while implementing the driver some feedback for the specification was
generated to cover perceived gaps and address conflicts. The feedback is
presented as a reference implementation in the driver and QEMU emulation.
Specifically the following concepts are original to the Linux implementation and
feedback / collaboration is requested to develop these into specification
proposals:
1. Top level ACPI object (ACPI0017)
2. HW imposed address space and interleave constraints

ACPI0017
--------
Introduce a new ACPI namespace device with an _HID of ACPI0017. The purpose of
this object is twofold, support a legacy OS with a set of out-of-tree CXL
modules, and establish an attach point for a driver that knows about
interleaving. Both of these boil down to the same point, to centralize Operating
System support for resources described by the CXL Early Discovery Table (CEDT).

The legacy OS problem stems from the spec's description of a host bridge,
ACPI0016 is denoted as the _HID for host bridges, with a _CID of PNP0A08. In a
CXL unaware version of Linux, the core ACPI subsystem will bind a driver to
PNP0A08 and preclude a CXL-aware driver from binding to ACPI0016. An ACPI0017
device allows a standalone CXL-aware driver to register for handling /
coordinating CEDT and CXL-specific _OSC control.

Similarly when managing interleaving there needs to be some management layer
above the ACPI0016 device that is capable of assembling leaf nodes into
interleave sets. As is the case with ACPI0012 that does this central
coordination for NFIT defined resources, ACPI0017 does the same for CEDT
described resources.

Memory Windows
-------
For CXL.mem capable platforms, there is a need for a mechanism for platform
firmware to make the Operating System aware of any restrictions that hardware
might have in address space. For example, in a system with 4 host bridges all
participating in an interleave set, the firmware needs to provide some
description of this. That information is missing from the CXL 2.0 spec as of
today and it also is not implemented in the driver. A variety of ACPI based
mechanisms, for example _CRS fields on the ACPI0017 device, were considered.

Next steps after this basic foundation is expanded command support and LIBNVDIMM
integration. This is the initial “release early / release often” version of the
Linux CXL enabling.

[1]: https://lore.kernel.org/linux-cxl/20201209002418.1976362-1-ben.widawsky@xxxxxxxxx/
[2]: https://www.computeexpresslink.org/
[3]: https://lore.kernel.org/qemu-devel/20210105165323.783725-1-ben.widawsky@xxxxxxxxx/T/#t
[4]: https://gitlab.com/bwidawsk/qemu/-/tree/cxl-2.0v2

Ben Widawsky (12):
  docs: cxl: Add basic documentation
  cxl/mem: Map memory device registers
  cxl/mem: Find device capabilities
  cxl/mem: Implement polled mode mailbox
  cxl/mem: Add basic IOCTL interface
  cxl/mem: Add send command
  taint: add taint for direct hardware access
  cxl/mem: Add a "RAW" send command
  cxl/mem: Create concept of enabled commands
  cxl/mem: Use CEL for enabling commands
  cxl/mem: Add limited Get Log command (0401h)
  MAINTAINERS: Add maintainers of the CXL driver

Dan Williams (2):
  cxl/mem: Introduce a driver for CXL-2.0-Type-3 endpoints
  cxl/mem: Register CXL memX devices

Vishal Verma (2):
  cxl/acpi: Add an acpi_cxl module for the CXL interconnect
  cxl/acpi: add OSC support

 .clang-format                                 |    1 +
 Documentation/ABI/testing/sysfs-bus-cxl       |   26 +
 Documentation/admin-guide/sysctl/kernel.rst   |    1 +
 Documentation/admin-guide/tainted-kernels.rst |    6 +-
 Documentation/cxl/index.rst                   |   12 +
 Documentation/cxl/memory-devices.rst          |   51 +
 Documentation/index.rst                       |    1 +
 .../userspace-api/ioctl/ioctl-number.rst      |    1 +
 MAINTAINERS                                   |   10 +
 drivers/Kconfig                               |    1 +
 drivers/Makefile                              |    1 +
 drivers/base/core.c                           |   14 +
 drivers/cxl/Kconfig                           |   58 +
 drivers/cxl/Makefile                          |    9 +
 drivers/cxl/acpi.c                            |  351 ++++
 drivers/cxl/acpi.h                            |   35 +
 drivers/cxl/bus.c                             |   54 +
 drivers/cxl/bus.h                             |    8 +
 drivers/cxl/cxl.h                             |  147 ++
 drivers/cxl/mem.c                             | 1475 +++++++++++++++++
 drivers/cxl/pci.h                             |   34 +
 include/acpi/actbl1.h                         |   50 +
 include/linux/device.h                        |    1 +
 include/linux/kernel.h                        |    3 +-
 include/uapi/linux/cxl_mem.h                  |  168 ++
 kernel/panic.c                                |    1 +
 26 files changed, 2517 insertions(+), 2 deletions(-)
 create mode 100644 Documentation/ABI/testing/sysfs-bus-cxl
 create mode 100644 Documentation/cxl/index.rst
 create mode 100644 Documentation/cxl/memory-devices.rst
 create mode 100644 drivers/cxl/Kconfig
 create mode 100644 drivers/cxl/Makefile
 create mode 100644 drivers/cxl/acpi.c
 create mode 100644 drivers/cxl/acpi.h
 create mode 100644 drivers/cxl/bus.c
 create mode 100644 drivers/cxl/bus.h
 create mode 100644 drivers/cxl/cxl.h
 create mode 100644 drivers/cxl/mem.c
 create mode 100644 drivers/cxl/pci.h
 create mode 100644 include/uapi/linux/cxl_mem.h

-- 
2.30.0