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Subject[RFC, PATCH 0/24] VMI i386 Linux virtualization interface proposal
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In OLS 2005, we described the work that we have been doing in VMware
with respect a common interface for paravirtualization of Linux. We
shared the general vision in Rik's virtualization BoF.

This note is an update on our further work on the Virtual Machine
Interface, VMI. The patches provided have been tested on 2.6.16-rc6.
We are currently recollecting performance information for the new -rc6
kernel, but expect our numbers to match previous results, which showed
no impact whatsoever on macro benchmarks, and nearly neglible impact
on microbenchmarks.

Unlike the full-virtualization techniques used in the traditional VMware
products, paravirtualization is a technique where the operating system
is modified to enlighten the hypervisor with timely knowledge about the
operating system's activities. Since the hypervisor now depends on the
kernel to tell it about common idioms etc, it does not need to write
protect OS objects such as page and descriptor tables as a solution
based on full-virtualization needs. This has two important effects (a)
it shortens the critical path, since faulting is expensive on modern
processors (b) by eliminating complex heuristics the hypervisor is
simplified. While the former delivers performance, the latter is quite
important too.

Not surprisingly, paravirtualization's strength, ie that it encourages
tighter communication between the kernel and the hypervisor, is also its
weakness. Unless the changes to the operating system are moderated, you
can very quickly find yourself with a kernel that (a) looks and feels
like a brand new kernel or (b) cannot run on native machines or on newer
versions of the hypervisor without a full recompile. The former can
impede innovation in the Linux kernel, and the latter can be a problem
for software vendors.

VMware proposes VMI as a paravirtualization interface for Linux that
solves these problems.
- A VMI'fied Linux kernel runs unmodified on native hardware, and on
many hypervisors, while simultaneously delivering on the performance
promise of paravirtualization.
- VMI has a rich and low level interface, which allows the kernel to
cope with future hardware evolution by querying for hardware
capability. It is our expectation that a single kernel will run
unmodified on both today's processors with limited hardware
virtualization support and also keep up with any evolution on the
processor front
- VMI Linux is a fairly clean interface, with distinct name spaces
for objects from the kernel and the hypervisor. Nowhere do we mingle
names from the hypervisor with that of the kernel. This separation
allows innovation in the kernel to proceed at the same speed as
always. For most kernel developers, a VMI kernel looks and feels like
a regular Linux kernel.
- VMI Linux still supports "native" hypervisor device drivers, for
example a hypervisor vendor's own private network or block device
drivers which are free to use any interface desired to communicate
with the hypervisor.

At present, we are sharing a working implementation of the VMI for
2.6.16-rc6 version of Linux. We have verified that VMI Linux does indeed
run well on native machines (both P4 and Opterons), and on VMware style
hypervisors. VMI Linux has negligible overheads on native machines, so
much so, that we are confident that VMI Linux can, in the long run, be
the default Linux for i386. We believe that this interface is both
cleaner and more powerful than other proposals that have been made
towards virtualization of Linux, and can easily be adapted to work with
other hypervisors.

This is by no means finished work. A few of the areas that need more
attention and exploration are (a) 64bit support is still lacking, but we
feel a port of VMI to the 64 bit Linux can be done without too much
trouble (b) the Xen compatibility layer needs some work to bring it
up to the Xen 3.0 interfaces. Work is underway on this already, and
no major issues are expected at this time.

Two final notes. This is not an attempt to force a proprietary interface
into the Linux kernel. This is an attempt to find a common interface
that can be used by many hypervisors by isolating hypervisor specific
idioms into a neutral layer. This new layer is just what is claims to
be - a virtual machine interface, which allows hypervisor dependent code
to be abstracted in a way that benefits both Linux and hypervisor
development.

This is also not an attempt to define an exact and final specification
of how virtualization should be done in Linux. This is very much a work
in progress, and it is understood that the interfaces proposed here will
change in time to accommodate the needs of all interested parties. We
hope to find a common solution that can eventually become part of the
Linux kernel and serve as a model for other operating systems as well.

We appreciate your feedback on this design and the patches to Linux, and
welcome working with anyone who is interested in making virtualization
in Linux a friendly environment to innovate in. If you find the ideas
here interesting, please volunteer to help improve them.
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