Performance Counters

CV32E40P implements performance counters according to the RISC-V Privileged Specification, version 1.11 (see Hardware Performance Monitor, Section 3.1.11). The performance counters are placed inside the Control and Status Registers (CSRs) and can be accessed with the CSRRW(I) and CSRRS/C(I) instructions.

CV32E40P implements the clock cycle counter mcycle(h), the retired instruction counter minstret(h), as well as the parameterizable number of event counters mhpmcounter3(h) - mhpmcounter31(h) and the corresponding event selector CSRs mhpmevent3 - mhpmevent31, and the mcountinhibit CSR to individually enable/disable the counters. mcycle(h) and minstret(h) are always available.

All counters are 64 bit wide.

The number of event counters is determined by the parameter NUM_MHPMCOUNTERS with a range from 0 to 29 (default value of 1).

Unimplemented counters always read 0.

Note

All performance counters are using the gated version of clk_i. The wfi instruction, the cv.elw instruction, and pulp_clock_en_i impact the gating of clk_i as explained in Sleep Unit and can therefore affect the counters.

Event Selector

The following events can be monitored using the performance counters of CV32E40P.

Bit #	Event Name
0	CYCLES	Number of cycles
1	INSTR	Number of instructions retired
2	LD_STALL	Number of load use hazards
3	JMP_STALL	Number of jump register hazards
4	IMISS	Cycles waiting for instruction fethces, excluding jumps and branches
5	LD	Number of load instructions
6	ST	Number of store instructions
7	JUMP	Number of jumps (unconditional)
8	BRANCH	Number of branches (conditional)
9	BRANCH_TAKEN	Number of branches taken (conditional)
10	COMP_INSTR	Number of compressed instructions retired
11	PIPE_STALL	Cycles from stalled pipeline
12	APU_TYPE	Numbe of type conflicts on APU/FP
13	APU_CONT	Number of contentions on APU/FP
14	APU_DEP	Number of dependency stall on APU/FP
15	APU_WB	Number of write backs on APUB/FP

The event selector CSRs mhpmevent3 - mhpmevent31 define which of these events are counted by the event counters mhpmcounter3(h) - mhpmcounter31(h). If a specific bit in an event selector CSR is set to 1, this means that events with this ID are being counted by the counter associated with that selector CSR. If an event selector CSR is 0, this means that the corresponding counter is not counting any event.

Note

At most 1 bit should be set in an event selector. If multiple bits are set in an event selector, then the operation of the associated counter is undefined.

Controlling the counters from software

By default, all available counters are disabled after reset in order to provide the lowest power consumption.

They can be individually enabled/disabled by overwriting the corresponding bit in the mcountinhibit CSR at address 0x320 as described in the RISC-V Privileged Specification, version 1.11 (see Machine Counter-Inhibit CSR, Section 3.1.13). In particular, to enable/disable mcycle(h), bit 0 must be written. For minstret(h), it is bit 2. For event counter mhpmcounterX(h), it is bit X.

The lower 32 bits of all counters can be accessed through the base register, whereas the upper 32 bits are accessed through the h-register. Reads of all these registers are non-destructive.

Parametrization at synthesis time

The mcycle(h) and minstret(h) counters are always available and 64 bit wide.

The number of available event counters mhpmcounterX(h) can be controlled via the NUM_MHPMCOUNTERS parameter. By default NUM_MHPCOUNTERS set to 1.

An increment of 1 to the NUM_MHPCOUNTERS results in the addition of the following:

• 64 flops for mhpmcounterX

• 15 flops for mhpmeventX

• 1 flop for mcountinhibit[X]

• Adder and event enablement logic

Time Registers (time(h))

The user mode time(h) registers are not implemented. Any access to these registers will cause an illegal instruction trap. It is recommended that a software trap handler is implemented to detect access of these CSRs and convert that into access of the platform-defined mtime register (if implemented in the platform).