qemu instructions for manual

[Leo Famulari <leo@famulari.name>]

Hi everyone,

I recently started using GuixSD in a virtual machine in advance of
getting some dedicated hardware.

This process is not well-documented, at least for somebody who is new to
QEMU. I don't think our manual should explain how to use QEMU, but I do
think it should provide the necessary information to get from `guix
system vm-image` to a working GuixSD virtual machine.

I'm looking for feedback on my understanding of QEMU so that my
contribution to the manual is accurate

This is a 'run-vm.sh' script produced by `guix system vm` for a simple
system declaration:

#!/gnu/store/ibpm6n6706yimzr3967krkxi2ibxq5yh-bash-4.3.39/bin/sh
exec /gnu/store/jm6wvkhq87kwr2grlravx6ix3w63p996-qemu-2.4.0.1/bin/qemu-system-x86_64 \
-kernel /gnu/store/lcs3ncjam9xxkczy34rdn7mxn0y912g9-linux-libre-4.3.3/bzImage \
-initrd /gnu/store/1n5q21mc49pkb33ynf6nzrmx11vgp28r-system/initrd \
-append "--system=/gnu/store/1n5q21mc49pkb33ynf6nzrmx11vgp28r-system --load=/gnu/store/1n5q21mc49pkb33ynf6nzrmx11vgp28r-system/boot --root=/dev/vda1 " \
-enable-kvm \
-no-reboot \
-net nic,model=virtio \
-virtfs local,path="/gnu/store",security_model=none,mount_tag="TAG_gnu_store" \
-net user \
-serial stdio \
-vga std \
-drive file=/gnu/store/2azafsay573829m1hynp27kwvmssdp1f-qemu-image,if=virtio,cache=writeback,werror=report,readonly \
-m 256 \
"$@"

This appears to be the minimum QEMU invocation that works:
$ qemu-system-x86_64 \
-net user \
-net nic,model=virtio \
-m 256 \
./qemu-image

Is there anything else that can be adapted from the run-vm.sh script?

After playing around with `guix system vm-image`, this appears to be the
minimum list of commands that will successfully boot GuixSD:
$ guix system vm-image --image-size=1G ~/guix/doc/os-config-bare-bones.texi 
$ cp /gnu/store/...-qemu-image ./qemu-image && guix gc -d /gnu/store/...-qemu-image
$ chmod 600 ./qemu-image
$ qemu-system-x86_64 \
-net user \
-net nic,model=virtio \
-m 256 \
./qemu-image

Annotated version:
# System type to emulate. Does this have to match the host hardware? Can
# "foreign" arch machine images be built with Guix?
$ qemu-system-x86_64 \

# Unprivileged user mode networking. Guest can access host but not vice versa.
# If you don't choose a network stack, the boot process will fail.
-net user \

# You must create a network interface of a given model. You can get a list of
# available NIC types by running `qemu-system-$arch -net nic,model=help`. If
# you don't create a NIC, the boot process will fail.
-net nic,model=virtio \

# RAM available to the guest OS. Defaults to 128 megabytes, which is not enough
# for the Guix daemon. More is better.
-m 256 \

OPTIONAL: If your system is x86 with hardware virtualization extensions,
enabling the kernel virtual machine will make things go much faster.
-enable-kvm \

# Path to the virtual machine image
./qemu-image

Any feedback?