Micro-architectural Event Tracking
Version 1.9.1 of the RISC-V Privileged Architecture adds support for Hardware Performance Monitor (HPM) counters.  The HPM support allows a nearly infinite number of micro-architectural events (called Hardware Performance Events (HPEs)) to be multiplexed onto up to multiple physical counters (called Hardware Performance Counters (HPCs)).
Setup HPM events to track
The available HPE's are split into event sets and events. Event sets are groupings of similar microarchitectural events (branch prediction events, memory events, etc). To access an HPE you must choose the correct event set and event bit and write to the proper HPC register for that event. An example of event set numbers and the event bit for a particular event is given below.
|Event Set #||Event Bit||Description|
|1||4||Control Flow Target Mispredict|
To access an HPC, you must first set up the privilege access level
of the particular HPC using
you write to the particular HPC register to setup which event(s) you want to
track. Bits [7:0] of the HPC register correspond to the event set while bits [?:8]
correspond to the event bitmask. Note that the bitmask can be a
singular event or multiple events.
Reading HPM counters in software
The Code Example :numref:`read-csr` demonstrates how to read the value of
any HPC from software. Note that HPCs need to be "zero'd" out
by first reading the value at the beginning of the program, then reading the
counter again the end, and then subtracting the initial value from the second
read. However, this only applies to the HPC's not
Adding your own HPE
To add your own HPE, you modify the event set and particular event in
src/main/scala/exu/core.scala. Note that the 1st item in the
to the first bit in the event set.
Information in this section was adapted from https://static.dev.sifive.com/U54-MC-RVCoreIP.pdf which details more about HPE/C's from RocketChip's perspective. Note: The HPE's supported by Rocket and BOOM differ, but the mechanism to access them is the same!
|||Future efforts may add some counters into a memory-mapped access region. This will open up the ability to track events that, for example, may not be tied to any particular core (like last-level cache misses).|