boot

In order to save memory and boot time, QEMU 4.2 and later versions are able to mmap(2) the kernel and initrd specified with -kernel and -initrd parameters. This approach allows us to avoid reading and copying them into a buffer, saving memory and time.

The memory pages that contain kernel and initrd are shared between multiple VMs using the same kernel and initrd images. So, when many VMs are launched we can save memory by sharing pages, and save time by avoiding reading them each time from the disk.

The stefano-garzarella/qemu-boot-time repository contains a Python perf-script and (Linux, QEMU, SeaBIOS) patches to measure the boot time of a Linux VM with QEMU/KVM.

Using I/O writes, we can trace events to measure the time consumed during the boot phase by the different components:

• firmware
  • SeaBIOS
  • qboot
• optionrom
  • linuxboot [bzImage]
  • pvh [vmlinux + PVH entry point]
• Linux kernel

QEMU 4.0 is now able to boot directly into the uncompressed Linux x86_64 kernel binary with minimal firmware involvement using the PVH entry point defined in the x86/HVM direct boot ABI. (CONFIG_PVH=y must be enabled in the Linux config file).

The x86/HVM direct boot ABI was initially developed for Xen guests, but with latest changes in both QEMU and Linux, QEMU is able to use that same entry point for booting KVM guests.