qemu-basics-x86.md

Qemu Setup

Build from source

$ git clone https://git.qemu.org/git/qemu.git
$ cd qemu
$ mkdir -p build && cd build
$ ../configure --target-list=x86_64-softmmu
$ make -j$(nproc)

User Setup

Add user to kvm group and qemu binaries to path

$ sudo usermod -aG kvm $USER
$ echo -e "export PATH=$PATH:$HOME/qemu/build/:$HOME/qemu/build/x86_64-softmmu/" | tee -a $HOME/.profile

NOTE

A re-login is necessary for the changes to be applied.

Disk Setup

Create disk image

$ qemu-img create -f qcow2 vmdisk.qcow2 30G

Start installation

$ wget --trust-server-names --content-disposition https://download.fedoraproject.org/pub/fedora/linux/releases/35/Server/x86_64/iso/Fedora-Server-netinst-x86_64-35-1.2.iso
$ qemu-system-x86_64 -enable-kvm -m 8G -smp 8 -vga none -nographic -net nic,model=virtio -net user -drive file=vmdisk.qcow2 -cdrom Fedora-Server-netinst-x86_64-35-1.2.iso -serial mon:stdio

NOTE

Some distributions, like Fedora, will attempt to use a graphical installer. To avoid that, force usage of the serial console. Using -serial mon:stdio, the guest is provided access to a virtual serial device which redirects everything to the host console. Edit the grub entry that starts the installation and append console=ttyS0 to force the installer to continue in text mode. For more verbosity, add earlyprintk and remove quiet.

NOTE

Avoid using lvm partitioning to avoid complexities with running custom kernels in the guest.

Start guest

$ qemu-system-x86_64 -enable-kvm -m 8G -smp 8 -vga none -nographic -net nic,model=virtio -net user -drive file=vmdisk.qcow2 -serial mon:stdio

NOTE

All this does is remove the -cdrom option as installation media is no longer required.

Custom Kernel Testing

Start guest

$ qemu-system-x86_64 -enable-kvm -m 8G -smp 8 -kernel linux/arch/x86/boot/bzImage -append "root=/dev/vda2 ro console=ttyS0" -vga none -nographic -net nic,model=virtio -net user -device virtio-blk-pci,drive=vmdisk -drive file=vmdisk.qcow2,if=none,id=vmdisk -serial mon:stdio

NOTE

An initrd will not be used here. So make sure that all important modules are built into the vmlinux binary i.e. CONFIG_... is set to y instead of m. A good starting point is to capture lsmod output within the guest, save that to lsmod.txt on the host, and use that to create the custom kernel's .config.

$ make LSMOD="lsmod.txt" localyesconfig

NOTE

For network and disk access, CONFIG_VIRTIO_NET and CONFIG_VIRTIO_BLK must be set to y instead of m.

NOTE

SeaBIOS expects the bzImage as the payload for -kernel.

$ make -j$(nproc) vmlinux bzImage