Fuchsia’s hypervisor

It's hard to imagine launching a new mobile OS without support for existing apps. In regards to Fuchsia, I initially thought the Magenta kernel might be compatible with the Android user space. While the potential implementation details for Android support have not been clear, another possibility has become more apparent. I'm not saying anything specific will or will not happen, so please take the following with a grain of salt.

Before starting this blog I spotted some initial comments in Fuschia source about a hypervisor. There was extremely little mentioned, it seemed almost tangential or experimental, and honestly I thought, "nah, that wouldn't make any sense."

Google has since been developing a hypervisor as part of Magenta though. (The first commits were seemingly on the same day as my first blog post.) I have been hesitant to write anything, because you can virtualize anything.

This one difference could imply a ton. If this is what Google is doing, then Fuchsia really is a fully new OS, and I can understand why some people would take offense to the idea of it being a mashup of Android and Chrome OS. Significant amounts of code (Mojo) are derived from Chromium, however, and the ability to run Android apps in VMs could look roughly the same as user space compatibility would to users, at least superficially. Fuchsia will still end up gradually replacing Android and Chrome OS for consumers, while Chrome OS will live on for education.

In this scenario, Google would not be swapping kernels outright, but instead technically would be running two different kernels on a Fuchsia device. (I will again stress that the consumer definition of an OS is not just the kernel either.) The kernel underlying both Fuchsia and Android/Linux would be Magenta. Perhaps using a microkernel would make this a far more manageable or performant approach.

It is entirely possible to run virtual machines like containers, and many companies offer such solutions. Given such an implementation, it is possible to virtualize an individual app without providing an entire second desktop environment for users to manage, despite the app being run on a guest VM.

Hypervisors and containers are not new ideas whatsoever. Fuchsia’s implementation is a Type-1 hypervisor (bare metal) and is currently being built for x86 and MSM8998, which offer hardware-assisted virtualization commonly featured by CPUs. I have basically zero knowledge about virtualization, so there's not much more I can say beyond that.

Running Android on top of a hypervisor would make Android more of a legacy environment than a legacy API for Fuchsia, per se. It would also make Google’s statements that Chrome OS is not going away and that it wouldn’t make any sense for Android and Chrome OS to merge strictly true on a technical level. Again, this still means Fuchsia would crucially provide a stable driver ABI that Linux does not offer.

The massive upside to this approach would be that Magenta would be a clean slate free of the constraints of Linux, Unix, and possibly POSIX (to some degree?). I’m not a kernel expert, but I understand why this would be a huge deal resulting in numerous important technical differences vs. a *nix OS. I’m sure the Fuchsia team would stress the advantages of its decisions. Performance, efficiency, and a million other things could potentially be improved over Linux.

As for the downsides, wouldn’t a hypervisor significantly hurt mobile battery life? Performance and other functional tradeoffs also seem inevitable; virtualization is certainly not costless. But if the downsides are moderate, Fuchsia’s native performance is hopefully unaffected, and the need for virtualization is eliminated long term by Fuchsia replacing Android entirely, these seem like potentially reasonable costs. Without benchmarking, though, there is no hard data upon which to base an opinion.

Relatively seamless Android compatibility is a must in my opinion, because I don’t see how Fuchsia could be so much radically better on its own that consumers and developers would pounce on it otherwise. I’m sure there will be all sorts of carrots and sticks to incentivize writing Flutter apps, but it’s hard to picture Android developers re-writing all of their apps anytime soon. This is the same developer base that Google has struggled to even get to adopt something as crucial as the JobScheduler API to not waste users' battery life, which should have been the OS’ responsibility in the first place.

Google will probably first market Flutter heavily at I/O 2017 as a better, reactive, and cross-platform way of making apps, and then at some point add on “and they’ll run on Fuchsia as well!” It’s hard to imagine wild developer and device vendor enthusiasm for such an approach without making it clear that Android will eventually become legacy. This is due to the network effects implicit with technological platforms. (Unfortunately “network effects” is used as a hand-wavey phrase with little substance behind it, and no one in the tech industry seems familiar with the technical academic details from economics, but that is a discussion for another day.)

There are other considerations at play such as the AOSP vendor ecosystem, and I’m sure Google performed all due diligence in assessing its strategic technical options. There’s also still a lot in flux technically. Mojo IPC was changed, for example, and Mojo itself was seemingly absorbed into Magenta — Fuchsia’s API could end up being called anything regardless. Previous tricky questions such as “how will Fuchsia maintain compatibility with libraries like bionic?” would become irrelevant, though, if Android is simply virtualized. One remaining key question is how Fuchsia and Android apps would interact. This challenge seems extremely important and far from trivial.

As with any of my articles, technical corrections are both encouraged and highly appreciated.