CXL Performance Monitoring Unit (CPMU)

The CXL rev 3.0 specification provides a definition of CXL Performance Monitoring Unit in section 13.2: Performance Monitoring.

CXL components (e.g. Root Port, Switch Upstream Port, End Point) may have any number of CPMU instances. CPMU capabilities are fully discoverable from the devices. The specification provides event definitions for all CXL protocol message types and a set of additional events for things commonly counted on CXL devices (e.g. DRAM events).

CPMU driver

The CPMU driver registers a perf PMU with the name pmu_mem<X>.<Y> on the CXL bus representing the Yth CPMU for memX.


The associated PMU is registered as


In common with other CXL bus devices, the id has no specific meaning and the relationship to specific CXL device should be established via the device parent of the device on the CXL bus.

PMU driver provides description of available events and filter options in sysfs.

The "format" directory describes all formats of the config (event vendor id, group id and mask) config1 (threshold, filter enables) and config2 (filter parameters) fields of the perf_event_attr structure. The "events" directory describes all documented events show in perf list.

The events shown in perf list are the most fine grained events with a single bit of the event mask set. More general events may be enable by setting multiple mask bits in config. For example, all Device to Host Read Requests may be captured on a single counter by setting the bits for all of

  • d2h_req_rdcurr

  • d2h_req_rdown

  • d2h_req_rdshared

  • d2h_req_rdany

  • d2h_req_rdownnodata

Example of usage:

$#perf list
cxl_pmu_mem0.0/clock_ticks/                        [Kernel PMU event]
cxl_pmu_mem0.0/d2h_req_rdshared/                   [Kernel PMU event]
cxl_pmu_mem0.0/h2d_req_snpcur/                     [Kernel PMU event]
cxl_pmu_mem0.0/h2d_req_snpdata/                    [Kernel PMU event]
cxl_pmu_mem0.0/h2d_req_snpinv/                     [Kernel PMU event]

$# perf stat -a -e cxl_pmu_mem0.0/clock_ticks/ -e cxl_pmu_mem0.0/d2h_req_rdshared/

Vendor specific events may also be available and if so can be used via

$# perf stat -a -e cxl_pmu_mem0.0/vid=VID,gid=GID,mask=MASK/

The driver does not support sampling so "perf record" is unsupported. It only supports system-wide counting so attaching to a task is unsupported.