Linux Kernel Module Cheat

24.3.5. gem5 clock frequency

As of gem5 872cb227fdc0b4d60acc7840889d567a6936b6e1 defaults to 2GHz for fs.py:

    parser.add_option("--cpu-clock", action="store", type="string",
                      default='2GHz',
                      help="Clock for blocks running at CPU speed")

We can check that very easily by looking at the timestamps of a Exec trace of an gem5 AtomicSimpleCPU without any caches:

./run \
  --arch aarch64 \
  --emulator gem5 \
  --userland userland/arch/aarch64/freestanding/linux/hello.S \
  --trace-insts-stdout \
;

which shows:

      0: system.cpu: A0 T0 : @asm_main_after_prologue    :   movz   x0, #1, #0        : IntAlu :  D=0x0000000000000001  flags=(IsInteger)
    500: system.cpu: A0 T0 : @asm_main_after_prologue+4    :   adr   x1, #28            : IntAlu :  D=0x0000000000400098  flags=(IsInteger)
   1000: system.cpu: A0 T0 : @asm_main_after_prologue+8    :   ldr   w2, #4194464       : MemRead :  D=0x0000000000000006 A=0x4000a0  flags=(IsInteger|IsMemRef|IsLoad)
   1500: system.cpu: A0 T0 : @asm_main_after_prologue+12    :   movz   x8, #64, #0       : IntAlu :  D=0x0000000000000040  flags=(IsInteger)
   2000: system.cpu: A0 T0 : @asm_main_after_prologue+16    :   svc   #0x0               : IntAlu :   flags=(IsSerializeAfter|IsNonSpeculative|IsSyscall)
hello
   2500: system.cpu: A0 T0 : @asm_main_after_prologue+20    :   movz   x0, #0, #0        : IntAlu :  D=0x0000000000000000  flags=(IsInteger)
   3000: system.cpu: A0 T0 : @asm_main_after_prologue+24    :   movz   x8, #93, #0       : IntAlu :  D=0x000000000000005d  flags=(IsInteger)
   3500: system.cpu: A0 T0 : @asm_main_after_prologue+28    :   svc   #0x0               : IntAlu :   flags=(IsSerializeAfter|IsNonSpeculative|IsSyscall)

so we see that it runs one instruction every 500 ps which makes up 2GHz.

So if we change the frequency to say 1GHz and re-run it:

./run \
  --arch aarch64 \
  --emulator gem5 \
  --userland userland/arch/aarch64/freestanding/linux/hello.S \
  --trace-insts-stdout \
  -- \
  --cpu-clock 1GHz \
;

we get as expected:

      0: system.cpu: A0 T0 : @asm_main_after_prologue    :   movz   x0, #1, #0        : IntAlu :  D=0x0000000000000001  flags=(IsInteger)
   1000: system.cpu: A0 T0 : @asm_main_after_prologue+4    :   adr   x1, #28            : IntAlu :  D=0x0000000000400098  flags=(IsInteger)
   2000: system.cpu: A0 T0 : @asm_main_after_prologue+8    :   ldr   w2, #4194464       : MemRead :  D=0x0000000000000006 A=0x4000a0  flags=(IsInteger|IsMemRef|IsLoad)
   3000: system.cpu: A0 T0 : @asm_main_after_prologue+12    :   movz   x8, #64, #0       : IntAlu :  D=0x0000000000000040  flags=(IsInteger)
   4000: system.cpu: A0 T0 : @asm_main_after_prologue+16    :   svc   #0x0               : IntAlu :   flags=(IsSerializeAfter|IsNonSpeculative|IsSyscall)
hello
   5000: system.cpu: A0 T0 : @asm_main_after_prologue+20    :   movz   x0, #0, #0        : IntAlu :  D=0x0000000000000000  flags=(IsInteger)
   6000: system.cpu: A0 T0 : @asm_main_after_prologue+24    :   movz   x8, #93, #0       : IntAlu :  D=0x000000000000005d  flags=(IsInteger)
   7000: system.cpu: A0 T0 : @asm_main_after_prologue+28    :   svc   #0x0               : IntAlu :   flags=(IsSerializeAfter|IsNonSpeculative|IsSyscall)

As of gem5 872cb227fdc0b4d60acc7840889d567a6936b6e1, but like gem5 cache size, does not get propagated to the guest, and is not for example visible at:

ls /sys/devices/system/cpu/cpu0/cpufreq