commit: 4a5a81928eea773837240b167a4f8ed990f8f4f7
parent eec2a3865030436d4a812403539a1ec252347be3
Author: Drew DeVault <sir@cmpwn.com>
Date: Fri, 2 Sep 2022 11:54:49 +0200
In praise of qemu
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+---
+title: In praise of qemu
+date: 2022-09-02
+---
+
+[qemu][0] is another in a long line of great software started by [Fabrice
+Bellard][1]. It provides virtual machines for a wide variety of software
+architectures. Combined with KVM, it forms the foundation of nearly all cloud
+services, and it runs SourceHut in our self-hosted datacenters. Much like
+Bellard's ffmpeg revolutionized the multimedia software industry, qemu
+revolutionized virtualisation.
+
+[0]: https://www.qemu.org/
+[1]: https://bellard.org/
+
+[2]: https://www.kernel.org/doc/html/latest/admin-guide/binfmt-misc.html
+[3]: https://harelang.org/
+
+qemu comes with a large variety of studiously implemented virtual devices, from
+standard real-world hardware like e1000 network interfaces to accelerated
+virtual hardware like virtio drives. One can, with the right combination of
+command line arguments, produce a virtual machine of essentially any
+configuration, either for testing novel configurations or for running
+production-ready virtual machines. Network adapters, mouse & keyboard, IDE or
+SCSI or SATA drives, sound cards, graphics cards, serial ports — the
+works. Lower level, often arch-specific features, such as AHCI devices, SMP,
+NUMA, and so on, are also available and invaluable for testing any conceivable
+system configurations. And these configurations *work*, and work reliably.
+
+I have relied on this testing quite a bit when working on kernels, particularly
+on [my own Helios kernel][4]. With a little bit of command line magic, I can run
+a fully virtualised system with a serial driver connected to the parent
+terminal, with a hardware configuration appropriate to whatever I happen to be
+testing, in a manner such that running and testing my kernel is no different
+from running any other program. With -gdb I can set up gdb remote debugging and
+even debug my kernel as if it were a typical program. Anyone who remembers osdev
+in the Bochs days — or even earlier — understands the unprecedented
+luxury of such a development environment. Should I ever find myself working on a
+hardware configuration which is unsupported by qemu, my very first step will be
+patching qemu to support it. In my reckoning, qemu support is nearly as
+important for bringing up a new system as a C compiler is.
+
+[4]: https://drewdevault.com/2022/06/13/helios.html
+
+And qemu's implementation in C is simple, robust, and comprehensive. On the
+several occasions when I've had to read the code, it has been a pleasure.
+Furthermore, the comprehensive approach allows you to build out a virtualisation
+environment tuned precisely to your needs, whatever they may be, and it is
+accommodating of many needs. Sure, it's low level — running a qemu command
+line is certainly more intimidating than, say, VirtualBox — but the
+trade-off in power afforded to the user opens up innumerable use-cases that are
+simply not available on any other virtualisation platform.
+
+One of my favorite, lesser-known features of qemu is qemu-user, which allows you
+to register a [binfmt][2] handler to run executables compiled for an arbitrary
+architecture on Linux. Combined with a little chroot, this has made cross-arch
+development easier than it has ever been before, something I frequently rely on
+when working on [Hare][3]. If you do cross-architecture work and you haven't set
+up qemu-user yet, you're missing out.
+
+```
+$ uname -a
+Linux taiga 5.15.63-0-lts #1-Alpine SMP Fri, 26 Aug 2022 07:02:59 +0000 x86_64 GNU/Linux
+$ doas chroot roots/alpine-riscv64/ /bin/sh
+# uname -a
+Linux taiga 5.15.63-0-lts #1-Alpine SMP Fri, 26 Aug 2022 07:02:59 +0000 riscv64 Linux
+```
+
+<!-- Inline styles because lazy -->
+<small style="
+ text-align: center;
+ display: block;
+ color: #555;
+">This is amazing.</small>
+
+qemu also holds a special place in my heart as one of the first projects I
+contributed to over email 🙂 And they still use email today, and even [recommend
+SourceHut][5] to make the process easier for novices.
+
+[5]: https://qemu.readthedocs.io/en/v6.2.0/devel/submitting-a-patch.html#if-you-cannot-send-patch-emails
+
+So, to Fabrice, and the thousands of other contributors to qemu, I offer my
+thanks. qemu is one of my favorite pieces of software.
