Linux Kernel Module Cheat

24.6.4.1. gem5 fast forward

Besides switching CPUs after a checkpoint restore, fs.py also has the --fast-forward option to automatically run the script from the start on a less detailed CPU, and switch to a more detailed CPU at a given tick.

This is generally useless compared to checkpoint restoring because:

checkpoint restore allows to run multiple contents after the restore, and restoring to multiple different system states, which you almost always want to do
we generally don’t know the exact tick at which the region of interest will start, especially as the binaries change. It is much easier to just instrument the content with a checkoint m5op

But let’s give it a try anyway with userland/freestanding/gem5_checkpoint.S which was mentioned at gem5 checkpoint userland minimal example

./run \
  --arch aarch64 \
  --emulator gem5 \
  --static \
  --trace ExecAll,FmtFlag,O3CPU,SimpleCPU \
  --userland userland/freestanding/gem5_checkpoint.S \
  -- \
  --caches
  --cpu-type DerivO3CPU \
  --fast-forward 1000 \
;
cat "$(./getvar --arch aarch64 --emulator gem5 trace_txt_file)"

At gem5 2235168b72537535d74c645a70a85479801e0651 we see something like:

      0: O3CPU: system.switch_cpus: Creating O3CPU object.
      0: O3CPU: system.switch_cpus: Workload[0] process is 0      0: SimpleCPU: system.cpu: ActivateContext 0
      0: SimpleCPU: system.cpu.dcache_port: received snoop pkt for addr:0 WriteReq
      0: SimpleCPU: system.cpu.dcache_port: received snoop pkt for addr:0x40 WriteReq
...

      0: SimpleCPU: system.cpu.dcache_port: received snoop pkt for addr:0x1f92 WriteReq
      0: SimpleCPU: system.cpu.dcache_port: received snoop pkt for addr:0x1e40 WriteReq
      0: SimpleCPU: system.cpu.dcache_port: received snoop pkt for addr:0x1e30 WriteReq
      0: SimpleCPU: system.cpu: Tick
      0: ExecEnable: system.cpu: A0 T0 : @asm_main_after_prologue    :   movz   x0, #0, #0        : IntAlu :  D=0x0000000000000000  flags=(IsInteger)
    500: SimpleCPU: system.cpu: Tick
    500: ExecEnable: system.cpu: A0 T0 : @asm_main_after_prologue+4    :   movz   x1, #0, #0        : IntAlu :  D=0x0000000000000000  flags=(IsInteger)
   1000: SimpleCPU: system.cpu: Tick
   1000: ExecEnable: system.cpu: A0 T0 : @asm_main_after_prologue+8    :   m5checkpoint             : IntAlu :   flags=(IsInteger|IsNonSpeculative|IsUnverifiable)
   1000: O3CPU: system.switch_cpus: [tid:0] Calling activate thread.
   1000: O3CPU: system.switch_cpus: [tid:0] Adding to active threads list
   1500: O3CPU: system.switch_cpus:

FullO3CPU: Ticking main, FullO3CPU.
   1500: O3CPU: system.switch_cpus: Scheduling next tick!
   2000: O3CPU: system.switch_cpus:

FullO3CPU: Ticking main, FullO3CPU.
   2000: O3CPU: system.switch_cpus: Scheduling next tick!
   2500: O3CPU: system.switch_cpus:

...

FullO3CPU: Ticking main, FullO3CPU.
  44500: ExecEnable: system.switch_cpus: A0 T0 : @asm_main_after_prologue+12    :   movz   x0, #0, #0        : IntAlu :  D=0x00000000000
  48000: O3CPU: system.switch_cpus: Removing committed instruction [tid:0] PC (0x400084=>0x400088).(0=>1) [sn:1]
  48000: O3CPU: system.switch_cpus: Removing instruction, [tid:0] [sn:1] PC (0x400084=>0x400088).(0=>1)
  48000: O3CPU: system.switch_cpus: Scheduling next tick!
  48500: O3CPU: system.switch_cpus:

...

We can also compare that to the same log but without --fast-forward and other CPU switch options:

      0: SimpleCPU: system.cpu.dcache_port: received snoop pkt for addr:0x1e40 WriteReq
      0: SimpleCPU: system.cpu.dcache_port: received snoop pkt for addr:0x1e30 WriteReq
      0: SimpleCPU: system.cpu: Tick
      0: ExecEnable: system.cpu: A0 T0 : @asm_main_after_prologue    :   movz   x0, #0, #0        : IntAlu :  D=0x0000000000000000  flags=(IsInteger)
    500: SimpleCPU: system.cpu: Tick
    500: ExecEnable: system.cpu: A0 T0 : @asm_main_after_prologue+4    :   movz   x1, #0, #0        : IntAlu :  D=0x0000000000000000  flags=(IsInteger)
   1000: SimpleCPU: system.cpu: Tick
   1000: ExecEnable: system.cpu: A0 T0 : @asm_main_after_prologue+8    :   m5checkpoint             : IntAlu :   flags=(IsInteger|IsNonSpeculative|IsUnverifiable)
   1000: SimpleCPU: system.cpu: Resume
   1500: SimpleCPU: system.cpu: Tick
   1500: ExecEnable: system.cpu: A0 T0 : @asm_main_after_prologue+12    :   movz   x0, #0, #0        : IntAlu :  D=0x0000000000000000  flags=(IsInteger)
   2000: SimpleCPU: system.cpu: Tick
   2000: ExecEnable: system.cpu: A0 T0 : @asm_main_after_prologue+16    :   m5exit                   : No_OpClass :   flags=(IsInteger|IsNonSpeculative)

Therefore, it is clear that what we wanted happen:

up until the tick 1000, SimpleCPU was ticking
after tick 1000, cpu O3CPU started ticking

Bibliography:

https://cs.stackexchange.com/questions/69511/what-does-fast-forwarding-mean-in-the-context-of-cpu-simulation