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Gentoo Xen Linux Guide 1. Xen concepts Xen is designed to allow you to run multiple operating systems simultaneously on a single computer with performance close to that of a native operating system . In order to understand how this is possible some concepts need to be understood. In order to run two or more operating systems simultaneously on the same machine two basic problems must be overcome.
Probably the most difficult of these problems is Hardware isolation. There are two basic approaches to achieving hardware isolation, hardware emulation and hardware virtualization. Emulation involves presenting a fake representation of the hardware to the operating system. Emulation can allow you to run programs completely independently of the underlying hardware but it tends to be slow. The other approach, Virtualization attempts to use the hardware directly and arbitrate its access at the hardware level. This approach tends to be much faster but requires hardware support which is not currently found in consumer hardware. Xen takes an approach called para-virtualization that has performance similar to full virtualization, but that can be run on consumer level hardware. Para-virtualization uses a software layer below the OS to arbitrate hardware access and thus avoids the need for special hardware. OS's running under Xen need to be modified so that for operations requiring arbitration, they access the hypervisor rather then the hardware. In Xen OS instances (file system and associated kernel) are referred to as domains. A domain that has direct access to hardware is referred to as a privileged domain. In the default setup the base operating system (the one everything is controlled from) is a privileged domain and all other domains are unprivileged.
2. Gentoo in domain0 firstly you need to emerge the hypervisor and kernel source. The app-emulation/xen ebuild contains the hypervisor and a server for communicating with the hypervisor. The sys-kernel/xen-sources contains a Linux kernel modified to run under the Xen hypervisor.
you will need to configure your kernel for the hardware in your system. To do this change to the Xen sources directory then run make menuconfig.
This is the same as any other kernel except that there will be some extra options for Xen related features. It is OK to remove drivers for hardware you don't have but do not change any Xen options.
The following Xen settings are required for an domain0 kernel.
Because we will be using LVM you need to have the Device mapper support selected. you should also select the appropriate drivers for you hard drive.
You need to make sure the /dev file system support is not selected and that all the file systems you are using are selected (i.e. ext3). You should also make sure Advanced partition selection is selected
Now that your kernel is configured, it is time to compile and install it. Exit the menuconfig and start the compilation process:
When the kernel has finished compiling, copy the kernel image to /boot.
you will need to modify /boot/grub/grub.conf so the the hypervisor is run before the domain0 kernel is booted.
add the following entry to the /boot/grub/grub.conf file.
You should now reboot your system and check that the privileged domain0 boots. 3. Gentoo in domainN It is now time to setup the unprivileged domains. You will need to start by creating partitions for your new operating systems. I have chosen the simplest partition scheme, a single root partition and a swap file. The choice is mostly arbitrary. You can check out the Gentoo Linux x86 Handbook for other possible partitioning schemes.
Next we need to format the partitions. I have chosen to use ext3 but the choice is arbitrary.
The final stage is installing your OS on the new partition. This is mostly the same as a normal install. you will need to create the gentoo mount point and mount the root partition.
You will need to get a tarball with one of the stages and unpack it to your /mnt/gentoo directory. If you don't know how to do this read Gentoo Handbook chapter 5 Now unpack your downloaded stage onto your system. We use GNU's tar to proceed as it is the easiest method:
This will create the base folders. You will then need to bind the proc file system to the mount point on the new partition.
We will be using nfs to access portage so we will bind the portage partition from our main system. You will need to create the mount point then bind portage to it.
You should copy the /etc/resolve.conf file and the /etc/make.conf to /mnt/gentoo/etc
You should now chroot into the partition and continue the install process.
When you edit your fstab file it should contain the root file system and the swap file entries.
Set a static address because that is simple.
Next edit the hosts file and add the hostname.
In order for the os to be started you need to create a configuration file as follows.
Start the hypervisor access daemon and add it to the default domain.
You can now start the new domain and display the domain boot sequence.
the xm allows you to control the hypervisor. You can use it to start and stop domains, to check domain status and number of other hypervisor related functions. using the following command you can display the status of the guest domains.
using the following command you can stop a domain synchronously
4. Tips/Tricks/Thoughts/Hacks/etc tls does not work directly with Xen and has to be emulated. This makes tls libraries very slow. It is recommended that you disable tls for domains running under Xen.
5. References
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