Xen: The platform for OS research

A quote from the Xen homepage:
Xen is a virtual machine monitor for x86 that supports execution of multiple guest operating systems with unprecedented levels of performance and resource isolation. I assume that readers know what virtualization is so I'm not going to explain it further.
There are many interesting research operating systems, for example SPIN or Exokernel. Major weakness of these and other non-mainstream operating systems is hardware support: often they support little more but the hardware available at the time of development. Since writing device drivers is not an interesting part of OS research, they quickly get behind modern hardware.

Xen has the potential to change this situation. It can greatly lower the initial effort of writing much support OS code (boot-loaders, device-drivers, etc.) and prolong life time of the new OS.

It exposes a simple system interface to the guest OS. This interface hides much ugliness of the x86 architecture and to successfully run under Xen, your OS must use it instead of directly invoking privileged instructions.

But more importantly, Xen exposes very simple interface to the virtual disk and network "devices". Being simple, a driver for the guest OS is easy to code. This driver forwards the guest OS operations to the host OS. Currently supported host OS-es are Linux and NetBSD. Which means that if there is a driver for the physical disk or network card the host OS is using, your OS within Xen can also run on that hardware.

Linux and NetBSD are actively maintained and the OS has more time to gain popularity. Its authors are relieved of keeping pace with HW development, thus having more time to concentrate on the development of the core OS. And users don't have to fear that some time in the future they won't be able to run the OS on new hardware.

Some readers may object that the OS has much more HW to deal with than disk and network (e.g. sound and video). But you don't need to develop the core of the OS.

Added bonuses of using Xen for OS development:
  • The virtualized OS runs almost at the same speed as on bare hardware. According to this benchmark, the virtualized OS runs within 10% speed on bare hardware. Bochs or Qemu are much slower. The comparison is not quite fair because they emulate all of typical PC hardware. But as I've argued above, you rarely need this in OS development.
  • No slow reboots when the OS crashes in its development phase.
  • No need for writing much boiler-plate code such as boot loaders or switching to protected mode. Your "virtual CPU" starts in flat, paged protected mode.
  • Another project has developed a remote GDB stub which makes it possible to debug Xen and guest operating systems.
This article would not be complete without mentioning OSKit (quote)
...the OSKit's goal is to lower the barrier to entry to OS R&D and to lower its costs. The OSKit makes it vastly easier to create a new OS, port an existing OS to the x86...
Its development seems to have ceased in 2002 and seems to have the same problems as other research OS I've mentioned. It provides glue code to make it possible to use drivers from Linux and FreeBSD kernels. However, Linux and FreeBSD driver architecture is changing and it is questionable how many drivers for the modern hardware can be used.

Tags: OS Virtualization

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