23.9.5. QEMU record and replay
QEMU runs, unlike gem5, are not deterministic by default, however it does support a record and replay mechanism that allows you to replay a previous run deterministically.
This awesome feature allows you to examine a single run as many times as you would like until you understand everything:
# Record a run. ./run --eval-after './linux/rand_check.out;./linux/poweroff.out;' --record # Replay the run. ./run --eval-after './linux/rand_check.out;./linux/poweroff.out;' --replay
A convenient shortcut to do both at once to test the feature is:
./qemu-rr --eval-after './linux/rand_check.out;./linux/poweroff.out;'
By comparing the terminal output of both runs, we can see that they are the exact same, including things which normally differ across runs:
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timestamps of dmesg output
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rand_check.out output
The record and replay feature was revived around QEMU v3.0.0. In v5.2.0 it is quite usable, almost all peripherals and vCPUs are supported.
Documented at: https://github.com/qemu/qemu/blob/v5.2.0/docs/replay.txt
replay may be used with with network:
./qemu-rr --eval-after 'ifup -a;wget -S google.com;./linux/poweroff.out;'
arm and aarch64 targets can also be used with rr:
./qemu-rr --arch aarch64 --eval-after './linux/rand_check.out;./linux/poweroff.out;' ./qemu-rr --arch aarch64 --eval-after 'ifup -a;wget -S google.com;./linux/poweroff.out;'
Replay also supports initrd and no disk:
./build-buildroot --arch aarch64 --initrd ./qemu-rr --arch aarch64 --eval-after './linux/rand_check.out;./linux/poweroff.out;' --initrd