[SeaBIOS] SeaBIOS booting time optimization

[Samuel Ortiz]

Hi Stefano,

On Fri, Nov 16, 2018 at 05:13:59PM +0100, Stefano Garzarella wrote:
> Hi,
> I'm investigating the SeaBIOS booting time, to understand if we can reduce
> the boot time in some cases (e.g. legacy hardware is not needed). I think
> this
> can be interesting also for NEMU developers.
Definitely, thanks.


> Following this thread (
> https://mail.coreboot.org/pipermail/seabios/2015-July/009497.html),
> I'm using qboot (https://github.com/bonzini/qboot) to compare the SeaBIOS
> booting time.
> 
> As Paolo did in qboot, I manually add small debug port writes in SeaBIOS and
> linuxboot_dma.c (QEMU) to trace the events with perf (kvm_pio events)
When doing similar measuremnts, I also used the debug-exit qemu device
in order to make imho simpler measuremnts, see
https://github.com/intel/nemu/wiki/Measuring-Boot-Latency

Probably not as precised as yours, but it was enough for what we tried
to characterize.

> The goal is to have only one image of SeaBIOS configurable at runtime to
> reduce
> the boot time, avoiding unnecessary initialization.
> 
> Any pointers or suggestions would be helpful.
> 
> Following I put some preliminary measurements that I obtained:
> I used this QEMU command line:
> ./qemu-system-x86_64 -bios $BIOS -m 1G -cpu host -M accel=kvm \
> -kernel /boot/vmlinuz-4.18.18-300.fc29.x86_64 -append 'console=ttyS0' \
> -nographic -serial mon:stdio
> 
> For each test, I measured these times (in milliseconds) relative to the
> "sched_process_exec" event:
> - qemu_init_end: first kvm_entry (i.e. QEMU initialized has finished)
> - fw_start: first entry of the BIOS
> - fw_do_boot: after the BIOS initialization (e.g. PCI setup, etc.)
> - linux_start_boot: before the jump to the Linux kernel
Are you planning to also measure the total time to userspace as well?
How we set the hardware up from the FW/BIOS can also influence the
overall (up to ring 3) boot latency on our experiments.

> # qboot
> BIOS=/home/stefano/repos/qboot/bios.bin
>  qemu_init_end: 40.561234
>  fw_start: 40.721729 (+0.160495)
>  fw_do_boot: 47.025591 (+6.303862)
>  linux_start_boot: 48.874112 (+1.848521)
> 
> # SeaBIOS with default configuration
> BIOS=/home/stefano/repos/seabios/out_default/bios.bin
>  qemu_init_end: 40.419451
>  fw_start: 40.639967 (+0.220516)
>  fw_do_boot: 886.668828 (+846.028861)
>  linux_start_boot: 889.723547 (+3.054719)
> 
> # SeaBIOS with Kevin's configuration
> # (https://mail.coreboot.org/pipermail/seabios/2015-July/009508.html)
> # Note: this SeaBIOS setup is so stripped down that it can't actually boot
> an OS
> BIOS=/home/stefano/repos/seabios/out_kevin/bios.bin
>  qemu_init_end: 40.676412
>  fw_start: 40.755757 (+0.079345)
>  fw_do_boot: 56.427023 (+15.671266)
That's a slight improvement ;)


> I did the same tests also with NEMU (without using -M virt) and I have
> approximately the same results.
Yes, as expected.


> As the next step, I'll start from Kevin's configuration to have a minimal
> SeaBIOS image ables to boot a Linux kernel.
Please keep us posted.

Cheers,
Samuel.