In praise of qemu September 2, 2022 on Drew DeVault's blog

qemu is another in a long line of great software started by Fabrice Bellard. 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.

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. 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.

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 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. 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

This is amazing.

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 to make the process easier for novices.

So, to Fabrice, and the thousands of other contributors to qemu, I offer my thanks. qemu is one of my favorite pieces of software.

Articles from blogs I read Generated by openring

Go Developer Survey 2023 Q1 Results

An analysis of the results from the 2023 Q1 Go Developer Survey.

via The Go Blog May 11, 2023

Summary of changes for April 2023

Hey everyone! This is the list of all the changes we've done to our projects and apps during the month of April. We'll also be reporting in our on position in the world, and on our future plans. Summary Of Changes 100r.co, added charging electro…

via Hundred Rabbits May 1, 2023

HDR hackfest wrap-up

Last week I’ve been attending the HDR hackfest organized by Red Hat. The trip to Prague was rather chaotic: the morning right after the SourceHut DoS incident, I got a notification that my plane was cancelled, so had to re-book a new flight and hotel. Then a…

via emersion May 1, 2023