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RISC-V Instruction Set Manual, Volume I: RISC-V User-Level ISA , 20191214- December 2019

10 Counters

RISC-V ISAs provide a set of up to 32×64-bit performance counters and timers that are accessible via unprivileged XLEN read-only CSR registers 0xC000xC1F (with the upper 32 bits accessed via CSR registers 0xC800xC9F on RV32). The first three of these (CYCLE, TIME, and INSTRET) have dedicated functions (cycle count, real-time clock, and instructions-retired respectively), while the remaining counters, if implemented, provide programmable event counting.

10.1 Base Counters and Timers

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RV32I provides a number of 64-bit read-only user-level counters, which are mapped into the 12-bit CSR address space and accessed in 32-bit pieces using CSRRS instructions. In RV64I, the CSR instructions can manipulate 64-bit CSRs. In particular, the RDCYCLE, RDTIME, and RDINSTRET pseudoinstructions read the full 64 bits of the cycle, time, and instret counters. Hence, the RDCYCLEH, RDTIMEH, and RDINSTRETH instructions are not required in RV64I.

Some execution environments might prohibit access to counters to impede timing side-channel attacks.

The RDCYCLE pseudoinstruction reads the low XLEN bits of the cycle CSR which holds a count of the number of clock cycles executed by the processor core on which the hart is running from an arbitrary start time in the past. RDCYCLEH is an RV32I instruction that reads bits 63–32 of the same cycle counter. The underlying 64-bit counter should never overflow in practice. The rate at which the cycle counter advances will depend on the implementation and operating environment. The execution environment should provide a means to determine the current rate (cycles/second) at which the cycle counter is incrementing.

RDCYCLE is intended to return the number of cycles executed by the processor core, not the hart. Precisely defining what is a “core” is difficult given some implementation choices (e.g., AMD Bulldozer). Precisely defining what is a “clock cycle” is also difficult given the range of implementations (including software emulations), but the intent is that RDCYCLE is used for performance monitoring along with the other performance counters. In particular, where there is one hart/core, one would expect cycle-count/instructions-retired to measure CPI for a hart.

Cores don’t have to be exposed to software at all, and an implementor might choose to pretend multiple harts on one physical core are running on separate cores with one hart/core, and provide separate cycle counters for each hart. This might make sense in a simple barrel processor (e.g., CDC 6600 peripheral processors) where inter-hart timing interactions are non-existent or minimal.

Where there is more than one hart/core and dynamic multithreading, it is not generally possible to separate out cycles per hart (especially with SMT). It might be possible to define a separate performance counter that tried to capture the number of cycles a particular hart was running, but this definition would have to be very fuzzy to cover all the possible threading implementations. For example, should we only count cycles for which any instruction was issued to execution for this hart, and/or cycles any instruction retired, or include cycles this hart was occupying machine resources but couldn’t execute due to stalls while other harts went into execution? Likely, “all of the above” would be needed to have understandable performance stats. This complexity of defining a per-hart cycle count, and also the need in any case for a total per-core cycle count when tuning multithreaded code led to just standardizing the per-core cycle counter, which also happens to work well for the common single hart/core case.

Standardizing what happens during “sleep” is not practical given that what “sleep” means is not standardized across execution environments, but if the entire core is paused (entirely clock-gated or powered-down in deep sleep), then it is not executing clock cycles, and the cycle count shouldn’t be increasing per the spec. There are many details, e.g., whether clock cycles required to reset a processor after waking up from a power-down event should be counted, and these are considered execution-environment-specific details.

Even though there is no precise definition that works for all platforms, this is still a useful facility for most platforms, and an imprecise, common, “usually correct” standard here is better than no standard. The intent of RDCYCLE was primarily performance monitoring/tuning, and the specification was written with that goal in mind.

The RDTIME pseudoinstruction reads the low XLEN bits of the time CSR, which counts wall-clock real time that has passed from an arbitrary start time in the past. RDTIMEH is an RV32I-only instruction that reads bits 63–32 of the same real-time counter. The underlying 64-bit counter should never overflow in practice. The execution environment should provide a means of determining the period of the real-time counter (seconds/tick). The period must be constant. The real-time clocks of all harts in a single user application should be synchronized to within one tick of the real-time clock. The environment should provide a means to determine the accuracy of the clock.

On some simple platforms, cycle count might represent a valid implementation of RDTIME, but in this case, platforms should implement the RDTIME instruction as an alias for RDCYCLE to make code more portable, rather than using RDCYCLE to measure wall-clock time.

The RDINSTRET pseudoinstruction reads the low XLEN bits of the instret CSR, which counts the number of instructions retired by this hart from some arbitrary start point in the past. RDINSTRETH is an RV32I-only instruction that reads bits 63–32 of the same instruction counter. The underlying 64-bit counter should never overflow in practice.

The following code sequence will read a valid 64-bit cycle counter value into x3:x2, even if the counter overflows its lower half between reading its upper and lower halves.

Sample code for reading the 64-bit cycle counter in RV32.

We recommend provision of these basic counters in implementations as they are essential for basic performance analysis, adaptive and dynamic optimization, and to allow an application to work with real-time streams. Additional counters should be provided to help diagnose performance problems and these should be made accessible from user-level application code with low overhead.

We required the counters be 64 bits wide, even on RV32, as otherwise it is very difficult for software to determine if values have overflowed. For a low-end implementation, the upper 32 bits of each counter can be implemented using software counters incremented by a trap handler triggered by overflow of the lower 32 bits. The sample code described above shows how the full 64-bit width value can be safely read using the individual 32-bit instructions.

In some applications, it is important to be able to read multiple counters at the same instant in time. When run under a multitasking environment, a user thread can suffer a context switch while attempting to read the counters. One solution is for the user thread to read the real-time counter before and after reading the other counters to determine if a context switch occurred in the middle of the sequence, in which case the reads can be retried. We considered adding output latches to allow a user thread to snapshot the counter values atomically, but this would increase the size of the user context, especially for implementations with a richer set of counters.

10.2 Hardware Performance Counters

There is CSR space allocated for 29 additional unprivileged 64-bit hardware performance counters, hpmcounter3hpmcounter31. For RV32, the upper 32 bits of these performance counters is accessible via additional CSRs hpmcounter3h hpmcounter31h. These counters count platform-specific events and are configured via additional privileged registers. The number and width of these additional counters, and the set of events they count is platform-specific.

The privileged architecture manual describes the privileged CSRs controlling access to these counters and to set the events to be counted.

It would be useful to eventually standardize event settings to count ISA-level metrics, such as the number of floating-point instructions executed for example, and possibly a few common microarchitectural metrics, such as “L1 instruction cache misses”.