Implement performance counters and timers about rv32emu

[jserv]

Check the following materials in advance:

• PicoRV32: ENABLE_COUNTERS64 (default = 1)
  This parameter enables support for the RDCYCLEH, RDTIMEH, and RDINSTRETH instructions. If this parameter is set to 0, and ENABLE_COUNTERS is set to 1, then only the RDCYCLE, RDTIME, and RDINSTRET instructions are available.
• Mike Field's emulate-risc-v

[jserv]

fastrv32 is another reference for implementing timer stuff.

[jserv]

Writing my first assembly code.

Alternatively, you can check riscv-csr-access for inline assembly based routines.

[jserv]

Suppose that we are going to utilize a function getcycles:

uint64_t getcycles();

It can be implemented:

.text

.globl getcycles
.align 2
getcycles:
    csrr a1, mcycleh
    csrr a0, mcycle
    csrr a2, mcycleh
    bne a1, a2, getcycles
    ret
.size getcycles,.-getcycles

Similarly, function getinstret can be implemented as following:

.text

.globl getinstret
.align 2
getinstret:
    csrr a1, minstreth
    csrr a0, minstret
    csrr a2, minstreth
    bne a1, a2, getinstret
    ret
.size getinstret,.-getinstret