[Qemu-devel] Announcing qboot, a minimal x86 firmware for QEMU

[Paolo Bonzini <pbonzini@redhat.com>]

Some of you may have heard about the "Clear Containers" initiative from
Intel, which couple KVM with various kernel tricks to create extremely
lightweight virtual machines.  The experimental Clear Containers setup
requires only 18-20 MB to launch a virtual machine, and needs about 60
ms to boot.

Now, as all of you probably know, "QEMU is great for running Windows or
legacy Linux guests, but that flexibility comes at a hefty price. Not
only does all of the emulation consume memory, it also requires some
form of low-level firmware in the guest as well. All of this adds quite
a bit to virtual-machine startup times (500 to 700 milliseconds is not
unusual)".

Right?  In fact, it's for this reason that Clear Containers uses kvmtool
instead of QEMU.

No, wrong!  In fact, reporting bad performance is pretty much the same
as throwing down the gauntlet.

Enter qboot, a minimal x86 firmware that runs on QEMU and, together with
a slimmed-down QEMU configuration, boots a virtual machine in 40
milliseconds[2] on an Ivy Bridge Core i7 processor.

qboot is available at git://github.com/bonzini/qboot.git.  In all the
glory of its 8KB of code, it brings together various existing open
source components:

* a minimal (really minimal) 16-bit BIOS runtime based on kvmtool's own BIOS

* a couple hardware initialization routines written mostly from scratch
but with good help from SeaBIOS source code

* a minimal 32-bit libc based on kvm-unit-tests

* the Linux loader from QEMU itself

The repository has more information on how to achieve fast boot times,
and examples of using qboot.  Right now there is a limit of 8 MB for
vmlinuz+initrd+cmdline, which however should be enough for initrd-less
containers.

The first commit to qboot is more or less 24 hours old, so there is
definitely more work to do, in particular to extract ACPI tables from
QEMU and present them to the guest.  This is probably another day of
work or so, and it will enable multiprocessor guests with little or no
impact on the boot times.  SMBIOS information is also available from QEMU.

On the QEMU side, there is no support yet for persistent memory and the
NFIT tables from ACPI 6.0.  Once that (and ACPI support) is added, qboot
will automatically start using it.

Happy hacking!

Paolo