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diff --git a/content/2021-12-02-toying-with-virtio/index.md b/content/2021-12-02-toying-with-virtio/index.md new file mode 100644 index 0000000..c2ff031 --- /dev/null +++ b/content/2021-12-02-toying-with-virtio/index.md @@ -0,0 +1,373 @@ ++++ +title = "QEMU virtio configurations" + +[taxonomies] +tags = ["linux", "qemu", "virtio"] ++++ + +For my own reference I wanted to document some minimal [`virtio`][virtio] +device configurations with qemu and the required Linux kernel configuration to +enable those devices. + +The devices we will use are `virtio console`, `virtio blk` and `virtio net`. + +To make use of the virtio devices in qemu we are going to build and boot into +busybox based [`initramfs`][initramfs]. + +## Build initramfs + +For the initramfs there is not much magic, we will grab a copy of busybox, +configure it with the default config (`defconfig`) and enable static linking as +we will use it as rootfs. + +For the `init` process we will use the one provided by busybox but we have to +symlink it to `/init` as during boot, the kernel will extract the cpio +compressed initramfs into `rootfs` and look for the `/init` file. If that's not +found the kernel will fallback to an older mechanism an try to mount a root +partition (which we don't have). +> Optionally the init binary could be specified with the `rdinit=` kernel boot +> parameter. + +We populate the `/etc/inittab` and `/etc/init.d/rcS` with a minimal +configuration to mount the `proc`, `sys` and `dev` filesystems and drop into a +shell after the boot is completed. \ +Additionally we setup `/etc/passwd` and `/etc/shadow` with an entry for the +`root` user with the password `1234`, so we can login via the virtio console +later. + +```sh +{{ include_range(path="content/2021-12-02-toying-with-virtio/build_initramfs.sh", start=31, end=67) }} +``` + +The full build script is available under [build_initramfs.sh][build-initramfs]. + +## Virtio console + +To enable support for the virtio console we enable the kernel configs shown +below. +The pci configurations are enabled because in qemu the virtio console front-end +device (the one presented to the guest) is attached to the pci bus. + +```sh +{{ include_range(path="content/2021-12-02-toying-with-virtio/build_kernel.sh", start=32, end=38) }} +``` + +The full build script is available under [build_kernel.sh][build-kernel]. + +To boot-up the guest we use the following qemu configuration. + +```sh +qemu-system-x86_64 \ + -nographic \ + -cpu host \ + -enable-kvm \ + -kernel ./linux-$(VER)/arch/x86/boot/bzImage \ + -append "earlyprintk=ttyS0 console=ttyS0 root=/dev/ram0 ro" \ + -initrd ./initramfs.cpio.gz \ + -device virtio-serial-pci \ + -device virtconsole,chardev=vcon,name=console.0 \ + -chardev socket,id=vcon,ipv4=on,host=localhost,port=2222,server,telnet=on,wait=off +``` + +The important parts in this configuration are the last three lines. + +The `virtio-serial-pci` device creates the serial bus where the virtio console +is attached to. + +The `virtconsole` creates the virtio console device exposed to the guest +(front-end). The `chardev=vcon` option specifies that the chardev with +`id=vcon` is attached as back-end to the virtio console. +The back-end device is the one we will have access to from the host running the +emulation. + +The chardev back-end we configure to be a `socket`, running a telnet server +listening on port 2222. The `wait=off` tells qemu that it can directly boot +without waiting for a client connection. + +After booting the guest we are dropped into a shell and can verify that our +device is being detected properly. +```sh +root@virtio-box ~ # ls /sys/bus/virtio/devices/ +virtio0 +root@virtio-box ~ # cat /sys/bus/virtio/devices/virtio0/virtio-ports/vport0p0/name +console.0 +``` + +In `/etc/inittab`, we already configured to spawn `getty` on the first +hypervisor console `/dev/hvc0`. This will effectively run `login(1)` over the +serial console. + +From the host we can run `telnet localhost 2222` and are presented with a login shell to the guest. + +As we already included to launch `getty` on the first hypervisor console +`/dev/hvc0` in `/etc/inittab`, we can directly connect to the back-end chardev +and login to the guest with `root:1234`. + +```sh +> telnet -4 localhost 2222 +Trying 127.0.0.1... +Connected to localhost. +Escape character is '^]'. + +virtio-box login: root +Password: +root@virtio-box ~ # +``` + +## Virtio blk + +To enable support for the virtio block device we enable the kernel configs +shown below. +First we enable general support for block devices and then for virtio block +devices. Additionally we enable support for the `ext2` filesystem because we +are creating an ext2 filesystem to back the virtio block device. + +```sh +{{ include_range(path="content/2021-12-02-toying-with-virtio/build_kernel.sh", start=40, end=47) }} +``` + +The full build script is available under [build_kernel.sh][build-kernel]. + +Next we are creating the ext2 filesystem image. This we'll do by creating an +`128M` blob and format it with ext2 afterwards. Then we can mount the image +via a `loop` device and populate the filesystem. +```sh +{{ include_range(path="content/2021-12-02-toying-with-virtio/build_ext2.sh", start=3, end=7) }} +``` + +Before booting the guest we will attach the virtio block device to the VM. +Therefore we add the `-drive` configuration to our previous qemu invocation. + +```sh +qemu-system-x86_64 \ + ... + -drive if=virtio,file=fs.ext2,format=raw +``` + +The `-drive` option is a shortcut for a `-device (front-end) / -blockdev +(back-end)` pair. + +The `if=virtio` flag specifies the interface of the front-end device to be +`virtio`. + +The `file` and `format` flags configure the back-end to be a disk image. + +After booting the guest we are dropped into a shell and can verify a few +things. First we check if the virtio block device is detected, then we check if +we have support for the ext2 filesystem and finally we mount the disk. + +```sh +root@virtio-box ~ # ls -l /sys/block/ +lrwxrwxrwx 1 root 0 0 Dec 3 22:46 vda -> ../devices/pci0000:00/0000:00:05.0/virtio1/block/vda + +root@virtio-box ~ # cat /proc/filesystems +... + ext2 + +root@virtio-box ~ # mount -t ext2 /dev/vda /mnt +EXT2-fs (vda): warning: mounting unchecked fs, running e2fsck is recommended +ext2 filesystem being mounted at /mnt supports timestamps until 2038 (0x7fffffff) + +root@virtio-box ~ # cat /mnt/hello +world +``` + +## Virtio net + +To enable support for the virtio network device we enable the kernel configs +shown below. +First we enable general support for networking and TCP/IP and then enable the +core networking driver and the virtio net driver. + +```sh +{{ include_range(path="content/2021-12-02-toying-with-virtio/build_kernel.sh", start=49, end=62) }} +``` + +The full build script is available under [build_kernel.sh][build-kernel]. + +For the qemu device emulation we already decided on the front-end device, which +will be our virtio net device. \ +On the back-end we will choose the [`user`][qemu-user-net] option. This enables +a network stack implemented in userspace based on [libslirp][libslirp], which +has the benefit that we do not need to setup additional network interfaces and +therefore require any privileges. Fundamentally, [libslirp][libslirp] works by +replaying [Layer 2][osi-2] packets received from the guest NIC via the socket +API on the host ([Layer 4][osi-4]) and vice versa. User networking comes with a +set of limitations, for example +- Can not use `ping` inside the guest as `ICMP` is not supported. +- The guest is not accessible from the host. + +With the guest, qemu and the host in the picture this looks something like the +following. +``` ++--------------------------------------------+ +| host | +| +-------------------------+ | +| | guest | | +| | | | +| | user | | +| +------+------+-----------+ | +| | | eth0 | kernel | | +| | +--+---+ | | +| | | | | +| | +-----v--------+ | | +| | | nic (virtio) | | | +| +--+---+-----+--------+------+--+ | +| | | Layer 2 qemu | | +| | | (eth frames) | | +| | +----v-----+ | | +| | | libslirp | | | +| | +----+-----+ | | +| | | Layer 4 | | +| | | (socket API) | user | ++--+---------+--v---+--------------+---------+ +| | eth0 | kernel | +| +------+ | ++--------------------------------------------+ +``` + +The user networking implements a virtually NAT'ed sub-network with the address +range `10.0.2.0/24` running an internal dhcp server. By default, the dhcp +server assigns the following IP addresses which are interesting to us: +- `10.0.2.2` host running the qemu emulation +- `10.0.2.3` virtual DNS server +> The netdev options `net=addr/mask`, `host=addr`, `dns=addr` can be used to +> re-configure the sub-network (see [network options][qemu-nic-opts]). + +With the details of the sub-network in mind we can add some additional setup to +the initramfs which performs the basic network setup. + +We add the virtual DNS server to `/etc/resolv.conf` which will be used by the +libc resolver functions. + +Additionally we assign a static ip to the `eth0` network interface, bring the +interface up and define the default route via the host `10.0.2.2`. + +```sh +{{ include_range(path="content/2021-12-02-toying-with-virtio/build_initramfs.sh", start=69, end=85) }} +``` + +The full build script is available under [build_initramfs.sh][build-initramfs]. + +Before booting the guest we will attach the virtio net device and configure to +use the user network stack. +Therefore we add the `-nic` configuration to our previous qemu invocation. + +```sh +qemu-system-x86_64 \ + ... + -nic user,model=virtio-net-pci +``` + +The `-nic` option is a shortcut for a `-device (front-end) / -netdev +(back-end)` pair. + +After booting the guest we are dropped into a shell and can verify a few +things. First we check if the virtio net device is detected. Then we check if +the interface got configured and brought up correctly. + +```sh +root@virtio-box ~ # ls -l /sys/class/net/ +lrwxrwxrwx 1 root 0 0 Dec 4 16:56 eth0 -> ../../devices/pci0000:00/0000:00:03.0/virtio0/net/eth0 +lrwxrwxrwx 1 root 0 0 Dec 4 16:56 lo -> ../../devices/virtual/net/lo + + +root@virtio-box ~ # ip -o a +2: eth0 inet 10.0.2.15/24 scope global eth0 ... + +root@virtio-box ~ # ip route +default via 10.0.2.2 dev eth0 +10.0.2.0/24 dev eth0 scope link src 10.0.2.15 +``` + +We can resolve out domain and see that the virtual DNS gets contacted. + +```sh +root@virtio-box ~ # nslookup memzero.de +Server: 10.0.2.3 +Address: 10.0.2.3:53 + +Non-authoritative answer: +Name: memzero.de +Address: 46.101.148.203 +``` + +Additionally we can try to access a service running on the host. Therefore we +run a simple http server on the host (where we launched qemu) with the +following command `python3 -m http.server --bind 0.0.0.0 1234`. This will +launch the server to listen for any incoming address at port `1234`. + +From within the guest we can manually craft a simple http `GET` request and +send it to the http server running on the host. For that we use the IP address +`10.0.2.2` which the dhcp assigned to our host. + +```sh +root@virtio-box ~ # echo "GET / HTTP/1.0" | nc 10.0.2.2 1234 +HTTP/1.0 200 OK +Server: SimpleHTTP/0.6 Python/3.9.7 +Date: Sat, 04 Dec 2021 16:58:56 GMT +Content-type: text/html; charset=utf-8 +Content-Length: 917 + +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> +<html> +<head> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<title>Directory listing for /</title> +</head> +<body> +<h1>Directory listing for /</h1> +<hr> +<ul> +<li><a href="build_initramfs.sh">build_initramfs.sh</a></li> +... +</ul> +<hr> +</body> +</html> +``` + +## Appendix: Workspace + +To re-produce the setup and play around with it just grab a copy of the +following files: +- [Dockerfile][dockerfile] +- [Makefile][makefile] +- [build_initramfs.sh][build-initramfs] +- [build_kernel.sh][build-kernel] +- [build_ext2.sh][build-ext2] + +Then run the following steps to build everything. The prefix `[H]` and `[C]` +indicate whether this command is run on the host or inside the container +respectively. +```sh +# To see all the make targets. +[H] make help + +# Build docker image, start a container with the current working dir +# mounted. On the first invocation this takes some minutes to build +# the image. +[H]: make docker + +# Build kernel and initramfs. +[C]: make + +# Build ext2 fs as virtio blkdev backend. +[H]: make ext2 + +# Start qemu guest. +[H]: make run +``` + +[build-initramfs]: https://git.memzero.de/blog/tree/content/2021-12-02-toying-with-virtio/build_initramfs.sh?h=main +[build-kernel]: https://git.memzero.de/blog/tree/content/2021-12-02-toying-with-virtio/build_kernel.sh?h=main +[build-ext2]: https://git.memzero.de/blog/tree/content/2021-12-02-toying-with-virtio/build_ext2.sh?h=main +[makefile]: https://git.memzero.de/blog/tree/content/2021-12-02-toying-with-virtio/Makefile?h=main +[dockerfile]: https://git.memzero.de/blog/tree/content/2021-12-02-toying-with-virtio/Dockerfile?h=main +[initramfs]: https://www.kernel.org/doc/Documentation/filesystems/ramfs-rootfs-initramfs.txt +[virtio]: http://docs.oasis-open.org/virtio/virtio/v1.1/virtio-v1.1.pdf +[qemu-nic-opts]: https://www.qemu.org/docs/master/system/invocation.html#hxtool-5 +[qemu-user-net]: https://www.qemu.org/docs/master/system/devices/net.html#using-the-user-mode-network-stack +[libslirp]: https://gitlab.com/qemu-project/libslirp +[osi-2]: https://osi-model.com/data-link-layer +[osi-4]: https://osi-model.com/transport-layer |