A git tree of this series can be found here:
https://github.com/weiny2/linux-kernel/tree/dcd-v6-2025-04-13
This is now based on 6.15-rc2.
Due to the stagnation of solid requirements for users of DCD I do not
plan to rev this work in Q2 of 2025 and possibly beyond.
It is anticipated that this will support at least the initial
implementation of DCD devices, if and when they appear in the ecosystem.
The patch set should be reviewed with the limited set of functionality in
mind. Additional functionality can be added as devices support them.
It is strongly encouraged for individuals or companies wishing to bring
DCD devices to market review this set with the customer use cases they
have in mind.
Series info
===========
This series has 2 parts:
Patch 1-17: Core DCD support
Patch 18-19: cxl_test support
Background
==========
A Dynamic Capacity Device (DCD) (CXL 3.1 sec 9.13.3) is a CXL memory
device that allows memory capacity within a region to change
dynamically without the need for resetting the device, reconfiguring
HDM decoders, or reconfiguring software DAX regions.
One of the biggest anticipated use cases for Dynamic Capacity is to
allow hosts to dynamically add or remove memory from a host within a
data center without physically changing the per-host attached memory nor
rebooting the host.
The general flow for the addition or removal of memory is to have an
orchestrator coordinate the use of the memory. Generally there are 5
actors in such a system, the Orchestrator, Fabric Manager, the Logical
device, the Host Kernel, and a Host User.
An example work flow is shown below.
Orchestrator FM Device Host Kernel Host User
| | | | |
|-------------- Create region ------------------------>|
| | | | |
| | | |<-- Create ----|
| | | | Region |
| | | |(dynamic_ram_a)|
|<------------- Signal done ---------------------------|
| | | | |
|-- Add ----->|-- Add --->|--- Add --->| |
| Capacity | Extent | Extent | |
| | | | |
| |<- Accept -|<- Accept -| |
| | Extent | Extent | |
| | | |<- Create ---->|
| | | | DAX dev |-- Use memory
| | | | | |
| | | | | |
| | | |<- Release ----| <-+
| | | | DAX dev |
| | | | |
|<------------- Signal done ---------------------------|
| | | | |
|-- Remove -->|- Release->|- Release ->| |
| Capacity | Extent | Extent | |
| | | | |
| |<- Release-|<- Release -| |
| | Extent | Extent | |
| | | | |
|-- Add ----->|-- Add --->|--- Add --->| |
| Capacity | Extent | Extent | |
| | | | |
| |<- Accept -|<- Accept -| |
| | Extent | Extent | |
| | | |<- Create -----|
| | | | DAX dev |-- Use memory
| | | | | |
| | | |<- Release ----| <-+
| | | | DAX dev |
|<------------- Signal done ---------------------------|
| | | | |
|-- Remove -->|- Release->|- Release ->| |
| Capacity | Extent | Extent | |
| | | | |
| |<- Release-|<- Release -| |
| | Extent | Extent | |
| | | | |
|-- Add ----->|-- Add --->|--- Add --->| |
| Capacity | Extent | Extent | |
| | | |<- Create -----|
| | | | DAX dev |-- Use memory
| | | | | |
|-- Remove -->|- Release->|- Release ->| | |
| Capacity | Extent | Extent | | |
| | | | | |
| | | (Release Ignored) | |
| | | | | |
| | | |<- Release ----| <-+
| | | | DAX dev |
|<------------- Signal done ---------------------------|
| | | | |
| |- Release->|- Release ->| |
| | Extent | Extent | |
| | | | |
| |<- Release-|<- Release -| |
| | Extent | Extent | |
| | | |<- Destroy ----|
| | | | Region |
| | | | |
Implementation
==============
This series requires the creation of regions and DAX devices to be
closely synchronized with the Orchestrator and Fabric Manager. The host
kernel will reject extents if a region is not yet created. It also
ignores extent release if memory is in use (DAX device created). These
synchronizations are not anticipated to be an issue with real
applications.
Only a single dynamic ram partition is supported (dynamic_ram_a). The
requirements, use cases, and existence of actual hardware devices to
support more than one DC partition is unknown at this time. So a less
complex implementation was chosen.
In order to allow for capacity to be added and removed a new concept of
a sparse DAX region is introduced. A sparse DAX region may have 0 or
more bytes of available space. The total space depends on the number
and size of the extents which have been added.
It is anticipated that users of the memory will carefully coordinate the
surfacing of capacity with the creation of DAX devices which use that
capacity. Therefore, the allocation of the memory to DAX devices does
not allow for specific associations between DAX device and extent. This
keeps allocations of DAX devices similar to existing DAX region
behavior.
To keep the DAX memory allocation aligned with the existing DAX devices
which do not have tags, extents are not allowed to have tags in this
implementation. Future support for tags can be added when real use
cases surface.
Great care was taken to keep the extent tracking simple. Some xarray's
needed to be added but extra software objects are kept to a minimum.
Region extents are tracked as sub-devices of the DAX region. This
ensures that region destruction cleans up all extent allocations
properly.
The major functionality of this series includes:
- Getting the dynamic capacity (DC) configuration information from cxl
devices
- Configuring a DC partition found in hardware.
- Enhancing the CXL and DAX regions for dynamic capacity support
a. Maintain a logical separation between hardware extents and
software managed extents. This provides an abstraction
between the layers and should allow for interleaving in the
future
- Get existing hardware extent lists for endpoint decoders upon region
creation.
- Respond to DC capacity events and adjust available region memory.
a. Add capacity Events
b. Release capacity events
- Host response for add capacity
a. do not accept the extent if:
If the region does not exist
or an error occurs realizing the extent
b. If the region does exist
realize a DAX region extent with 1:1 mapping (no
interleave yet)
c. Support the event more bit by processing a list of extents
marked with the more bit together before setting up a
response.
- Host response for remove capacity
a. If no DAX device references the extent; release the extent
b. If a reference does exist, ignore the request.
(Require FM to issue release again.)
c. Release extents flagged with the 'more' bit individually as
the specification allows for the asynchronous release of
memory and the implementation is simplified by doing so.
- Modify DAX device creation/resize to account for extents within a
sparse DAX region
- Trace Dynamic Capacity events for debugging
- Add cxl-test infrastructure to allow for faster unit testing
(See new ndctl branch for cxl-dcd.sh test[1])
- Only support 0 value extent tags
Fan Ni's upstream of Qemu DCD was used for testing.
Remaining work:
1) Allow mapping to specific extents (perhaps based on
label/tag)
1a) devise region size reporting based on tags
2) Interleave support
Possible additional work depending on requirements:
1) Accept a new extent which extends (but overlaps) already
accepted extent(s)
2) Rework DAX device interfaces, memfd has been explored a bit
3) Support more than 1 DC partition
[1] https://github.com/weiny2/ndctl/tree/dcd-region3-2025-04-13
---
Changes in v9:
- djbw: pare down support to only a single DC parition
- djbw: adjust to the new core partition processing which aligns with
new type2 work.
- iweiny: address smaller comments from v8
- iweiny: rebase off of 6.15-rc1
- Link to v8:
https://patch.msgid.link/20241210-dcd-type2-upstream-v8-0-812852504...@intel.com
---
Ira Weiny (19):
cxl/mbox: Flag support for Dynamic Capacity Devices (DCD)
cxl/mem: Read dynamic capacity configuration from the device
cxl/cdat: Gather DSMAS data for DCD partitions
cxl/core: Enforce partition order/simplify partition calls
cxl/mem: Expose dynamic ram A partition in sysfs
cxl/port: Add 'dynamic_ram_a' to endpoint decoder mode
cxl/region: Add sparse DAX region support
cxl/events: Split event msgnum configuration from irq setup
cxl/pci: Factor out interrupt policy check
cxl/mem: Configure dynamic capacity interrupts
cxl/core: Return endpoint decoder information from region search
cxl/extent: Process dynamic partition events and realize region extents
cxl/region/extent: Expose region extent information in sysfs
dax/bus: Factor out dev dax resize logic
dax/region: Create resources on sparse DAX regions
cxl/region: Read existing extents on region creation
cxl/mem: Trace Dynamic capacity Event Record
tools/testing/cxl: Make event logs dynamic
tools/testing/cxl: Add DC Regions to mock mem data
Documentation/ABI/testing/sysfs-bus-cxl | 100 ++-
drivers/cxl/core/Makefile | 2 +-
drivers/cxl/core/cdat.c | 11 +
drivers/cxl/core/core.h | 33 +-
drivers/cxl/core/extent.c | 495 +++++++++++++++
drivers/cxl/core/hdm.c | 13 +-
drivers/cxl/core/mbox.c | 632 ++++++++++++++++++-
drivers/cxl/core/memdev.c | 87 ++-
drivers/cxl/core/port.c | 5 +
drivers/cxl/core/region.c | 76 ++-
drivers/cxl/core/trace.h | 65 ++
drivers/cxl/cxl.h | 61 +-
drivers/cxl/cxlmem.h | 134 +++-
drivers/cxl/mem.c | 2 +-
drivers/cxl/pci.c | 115 +++-
drivers/dax/bus.c | 356 +++++++++--
drivers/dax/bus.h | 4 +-
drivers/dax/cxl.c | 71 ++-
drivers/dax/dax-private.h | 40 ++
drivers/dax/hmem/hmem.c | 2 +-
drivers/dax/pmem.c | 2 +-
include/cxl/event.h | 31 +
include/linux/ioport.h | 3 +
tools/testing/cxl/Kbuild | 3 +-
tools/testing/cxl/test/mem.c | 1021 +++++++++++++++++++++++++++----
25 files changed, 3102 insertions(+), 262 deletions(-)
---
base-commit: 8ffd015db85fea3e15a77027fda6c02ced4d2444
change-id: 20230604-dcd-type2-upstream-0cd15f6216fd
Best regards,
--
Ira Weiny <ira.we...@intel.com>
