Dakshina`s Blog

Random notes on virtualization ...

Just am jotting down what i understand ... Have had some exposure to virtualization technologies in the past few months.

Virtualization allows a user to run multiple OS'es simultaeously on a single system,in a secure way. On x86 processors, when running in protected mode, there are four privilege levels.

The operating system kernel executes in privilege level 0 (also called "supervisor mode") while applications execute in privilege level 3. Privilege levels 1 and 2 are not used.

When using virtual machine extensions, there are two classes of software: VMM (Virtual Machine Monitor), also known as "hypervisor", and Guests, which are virtual machines. If we consider Xen ,we call it Domain0 and Guest domains(DomU's or unpriveleged domains).

With Sun's LDOMs they are known as Control domain and guest domain respectively.

The VMM acts as a host and has a full access to the hardware. It also hosts the management software (like xend incase of Xen and LDOM manager in case of LDOMS),which is responsible for lifecycle management of the guest domains.

In the Xen project, running on x86 processors, the guest operating systems run in privilege level 1.

Xen 2.0 had initial support for paravirtualization,meaning that guest OS'es would have to be tweaked to run on top of the hypervisor. Xen 3.0 and above support both paravirtualization and full virtualization to leverage the inbuilt hardware support built into the Intel-VT-x and AMD pacifica processors.In either cases,application binaries would run unmodified and they would run in ring 3 .

With built-in virtualization extensionsin processors ,the guest operating system code stays unmodified. With Xen running on non-virtualized processors, there is a device model which is based on backend/frontend virtual drivers (also called "split drivers"). The backend is in domain 0, while the frontend is in the unprivileged domains. Only domain 0 has access to the hardware through the unmodified Linux drivers. When running on Intel VT-x or AMD SVM(Secure Virtual machines), we cannot use this IO model, because the guests run unmodified Linux kernels.

Posted at 03:20PM Aug 29, 2006 by dakshina in Sun  |  Comments[0]