A few weeks ago I took a look at Qboot, a minimal x86 firmware for QEMU which can boot in milliseconds. Qboot was written by Paolo Bonzini and dates from 2015 and there is an LWN article with the original announcement.
I tried it on my machine and it booted in QEMU (with kvm) in about 20ms, from entering Qboot to entering Linux. Very impressive! I was intrigued as to what makes it so fast.
There is another repo, qemu-boot-time by Stefan Garzarella, which provides an easy means to benchmark the boot. It uses perf events in Linux to detect the start and end of Qboot.
Using x86 post codes, I added the same to U-Boot. Initially the boot time was 2.9 seconds! Terrible. Here is a script which works on my machine and measures the time taken for U-Boot boot, using the qemu-x86_64 target:
It turned out that almost two of the seconds were the U-Boot boot delay. Another 800ms was the PXE menu delay. With those removed the time dropped to 210ms, which is not bad.
Using CONFIG_NO_NET dropping CONFIG_VIDEO each shaved off about 50ms. I then tried passing the kernel and initrd through QEMU using the QFW interface. It only saved 15ms but it is something.
I figured that command-line processing would be quite slow. With CONFIG_CMDLINE disabled another 5ms was saved. A final 7ms came from disabling filesystems and EFI loader. Small gains.
In the end, my result is about 83ms (in bold below):
$ ./contrib/qemu-boot-timer.sh
starting perf
building U-Boot
running U-Boot in QEMU
waiting for a bit
qemu-system-x86_64: terminating on signal 15 from pid 2775874 (bash)
parsing perf results
1) pid 2779434
qemu_init_end: 51.924873
u_boot_start: 51.962744 (+0.037871)
u_boot_do_boot: 134.781048 (+82.818304)
One final note: the qemu-x86_64 target actually boots by starting SPL in 16-bit mode and then moving to 64-bit mode to start U-Boot proper. This was partly to avoid calling 16-bit video ROMs from 64-bit code. Now that bochs is used for the display, it should be easy enough to drop SPL for this target. I’m not sure how much time that would save.
Note: Some final hacks are tagged here.