QEMU 4.0 boots uncompressed Linux x86_64 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.

Prerequisites

  • QEMU >= 4.0
  • Linux kernel >= 4.21
    • CONFIG_PVH=y enabled
    • vmlinux uncompressed image

How to use

To boot the PVH kernel image, you can use the -kernel parameter specifying the path to the vmlinux image.

qemu-system-x86_64 -machine q35,accel=kvm \
    -kernel /path/to/vmlinux \
    -drive file=/path/to/rootfs.ext2,if=virtio,format=raw \
    -append 'root=/dev/vda console=ttyS0' -vga none -display none \
    -serial mon:stdio

The -initrd and -append parameters are also supported as for compressed images.

Details

QEMU will automatically recognize if the vmlinux image has the PVH entry point and it will use SeaBIOS with the new pvh.bin optionrom to load the uncompressed image into the guest VM.

As an alternative, qboot can be used to load the PVH image.

Performance

Perf script and patches used to measure boot time: https://github.com/stefano-garzarella/qemu-boot-time.

The following values are expressed in milliseconds [ms]

  • QEMU (q35 machine) + SeaBIOS + bzImage

    • qemu_init_end: 36.072056
    • linux_start_kernel: 114.669522 (+78.597466)
    • linux_start_user: 191.748567 (+77.079045)
  • QEMU (q35 machine) + SeaBIOS + vmlinux(PVH)

    • qemu_init_end: 51.588200
    • linux_start_kernel: 62.124665 (+10.536465)
    • linux_start_user: 139.460582 (+77.335917)
  • QEMU (q35 machine) + qboot + bzImage

    • qemu_init_end: 36.443638
    • linux_start_kernel: 106.73115 (+70.287512)
    • linux_start_user: 184.575531 (+77.844381)
  • QEMU (q35 machine) + qboot + vmlinux(PVH)

    • qemu_init_end: 51.877656
    • linux_start_kernel: 56.710735 (+4.833079)
    • linux_start_user: 133.808972 (+77.098237)
  • Tracepoints:

    • qemu_init_end: first kvm_entry (i.e. QEMU initialization has finished)
    • linux_start_kernel: first entry of the Linux kernel (start_kernel())
    • linux_start_user: before starting the init process

Patches

Linux patches merged upstream in Linux 4.21:

QEMU patches merged upstream in QEMU 4.0:

qboot patches merged upstream:

qemu  linux  boot 

See also