By Dan Williams

I'm excited to announce the release of Solo5! Solo5 is essentially a kernel library that bootstraps the hardware and forms a base (similar to Mini-OS) from which unikernels can be built. It runs on fully virtualized x86 hardware (e.g., KVM/QEMU), using virtio device interfaces.

Importantly, Solo5 is integrated (to some extent) with the MirageOS toolstack, so the Solo5 version of the Mirage toolstack can build Mirage unikernels that run directly on KVM/QEMU instead of Xen. As such, Solo5 can be considered an alternative to Mini-OS in the Mirage stack. Try it out today!

In the rest of this post, I'll give a bit of motivation about why I think the lowest layer of the unikernel is interesting and important, as well as a rough overview of the steps I took to create Solo5.

Why focus so far down the software stack?

When people think about Mirage unikernels, one of the first things that comes to mind is the use of a high-level language (OCaml). Indeed, the Mirage community has invested lots of time and effort producing implementations of traditional system components (e.g., an entire TCP stack) in OCaml. The pervasive use of OCaml contributes to security arguments for Mirage unikernels (strong type systems are good) and is an interesting design choice well worth exploring.

But underneath all of that OCaml goodness is a little kernel layer written in C. This layer has a direct impact on:

What environments the unikernel can run on. Mini-OS, for example, assumes a paravirtualized (Xen) machine, whereas Solo5 targets full x86 hardware virtualization with virtio devices.

Boot time. "Hardware" initialization (or lack of it in a paravirtualized case) is a major factor in achieving the 20 ms unikernel boot times that are changing the way people think about elasticity in the cloud.

Memory layout and protection. Hardware "features" like page-level write protection must be exposed by the lowest layer for techniques like memory tracing to be performed. Also, software-level strategies like address space layout randomization require cooperation of this lowest layer.

Low-level device interfacing. As individual devices (e.g., NICs) gain virtualization capabilities, the lowest software layer is an obvious place to interface directly with hardware.

Threads/events. The low-level code must ensure that device I/O is asynchronous and/or fits with the higher-level synchronization primitives.

The most popular existing code providing this low-level kernel layer is called Mini-OS. Mini-OS was (I believe) originally written as a vehicle to demonstrate the paravirtualized interface offered by Xen for people to have a reference to port their kernels to and as a base for new kernel builders to build specialized Xen domains. Mini-OS is a popular base for MirageOS, ClickOS, and other unikernels. Other software that implements a unikernel base include Rumprun and OSv.

I built Solo5 from scratch (rather than adapting Mini-OS, for example) primarily as an educational (and fun!) exercise to explore and really understand the role of the low-level kernel layer in a unikernel. To provide applications, Solo5 supports the Mirage stack. It is my hope that Solo5 can be a useful base for others; even if only at this point to run some Mirage applications on KVM/QEMU!

Solo5: Building a Unikernel Base from Scratch

At a high level, there are roughly 3 parts to building a unikernel base that runs on KVM/QEMU and supports Mirage:

Typical kernel hardware initialization. The kernel must know how to load things into memory at the desired locations and prepare the processor to operate in the correct mode (e.g., 64-bit). Unlike typical kernels, most setup is one-time and simplified. The kernel must set up a memory map, stack, interrupt vectors, and provide primitives for basic memory allocation. At its simplest, a unikernel base kernel does not need to worry about user address spaces, threads, or many other things typical kernels need.

Interact with virtio devices. virtio is a paravirtualized device standard supported by some hypervisors, including KVM/QEMU and Virtualbox. As far as devices go, virtio devices are simple: I was able to write (very simple/unoptimized) virtio drivers for Solo5 drivers from scratch in C. At some point it may be interesting to write them in OCaml like the Xen device drivers in Mirage, but for someone who doesn't know OCaml (like me) a simple C implementation seemed like a good first step. I should note that even though the drivers themselves are written in C, Solo5 does include some OCaml code to call out to the drivers so it can connect with Mirage.

Appropriately link Mirage binaries/build system. A piece of software called mirage-platform performs the binding between Mini-OS and the rest of the Mirage stack. Building a new unikernel base means that this "cut point" will have lots of undefined dependencies which can either be implemented in the new unikernel base, stubbed out, or reused. Other "cut points" involve device drivers: the console, network and block devices. Finally, the mirage tool needs to output appropriate Makefiles for the new target and an overall Makefile needs to put everything together.

Each one of these steps carries complexity and gotchas and I have certainly made many mistakes when performing all of them. The hardware initialization process is needlessly complex, and the overall Makefile reflects my ignorance of OCaml and its building and packaging systems. It's a work in progress!

Next Steps and Getting Involved

In addition to the aforementioned clean up, I'm currently exploring the boot time in this environment. So far I've found that generating a bootable iso with GRUB as a bootloader and relying on QEMU to emulate BIOS calls to load the kernel is, by the nature of emulation, inefficient and something that should be avoided.

If you find the lowest layer of the unikernel interesting, please don't hesitate to contact me or get involved. I've packaged the build and test environment for Solo5 into a Docker container to reduce the dependency burden in playing around with it. Check out the repo for the full instructions!

I'll be talking about Solo5 at the upcoming 2016 Unikernels and More: Cloud Innovators Forum event to be held on January 22, 2016 at SCALE 14X in Pasadena, CA USA. I look forward to meeting some of you there!

Discuss this post on devel.unikernel.org

Thanks to Amir, Mort, and Jeremy, for taking the time to read and comment on earlier drafts.