Tuesday, January 22, 2008

Quick HOWTO for building Qumranet's KVM

This post really deserves the name scrawl, but I thought sharing my experience in building a Qumranet's KVM snapshot on Linux could helpful for others. Indeed it was not as straightforward as it seemed at first glance and it required some grope and wandering on Google.

You will need KVM (surprising, isn't it?) and Linux kernel sources corresponding to your kernel.

FWIW, KVM snapshots can be downloaded here.

First, let's prepare the kernel tree. This step is only important for people who use packaged kernel (or IOW who don't build their kernel themselves and have extracted the kernel source tree for the sole purpose of building KVM). For others, this step could be avoided because the targets we will use are performed implicitely when building the kernel.

root# tar xzf linux-2.6.23.tar.gz
root# cd linux-2.6.23.tar.gz
root# cp /boot/config-2.6.23 .config
root# make oldconfig prepare scripts
root# cd -

Next we will build KVM. Beware your GCC version! The current major branch is GCC 4 and it is shipped with almost if not all recent Linux distributions. Unfortunately, QEMU (on which KVM's userland is heavily based, not to say they've reused QEMU with little modifications) doesn't build with GCC 4. It requires GCC 3.2 or GCC 3.4 (a thorough explanation is provided in Fabrice Bellard's paper, "QEMU, a fast and portable dynamic translator", USENIX 2005). So you need to install this version of GCC as well. If you are running Debian for instance, this is pretty straightforward :

root# aptitude install gcc-3.4

And you will have a new compiler named gcc-3.4. On other distros, you will have to either find a package of GCC 3 or build it manually as described on Gentoo Wiki.

Finally you will simply have to build KVM with a few special arguments to the configure script:

root# tar xzf kvm-snapshot-20080117.tar.gz
root# cd kvm-snapshot-20080117
root# ./configure --qemu-cc=gcc-3.4 --kerneldir=$OLDPWD/linux-2.6.23 --prefix=/opt/kvm-snapshot-20080117
root# make all install

And this should work. :-)

As you may have noticed, I installed KVM in /opt in order to avoid messing with your tidy package management system.

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