xen split driver model

iam confused over these two concepts. The xen split driver model and paravirtualization. Are these two the same ? 开发者_如何学运维Do you get the split driver model when xen is running in full virtualized mode ?

Paravirtualization is the general concept of making modifications to the kernel of a guest Operating System to make it aware that it is running on virtual, rather than physical, hardware, and so exploit this for greater efficiency or performance or security or whatever. A paravirtualized kernel may not function on physical hardware at all, in a similar fashion to attempting to run an Operating System on incompatible hardware.

The Split Driver model is one technique for creating efficient virtual hardware. One device driver runs inside the guest Virtual Machine (aka domU) and communicates with another corresponding device driver inside the control domain Virtual Machine (aka dom0). This pair of codesigned device drivers function together, and so can be considered to be a single "split" driver.

Examples of split device drivers are Xen's traditional block and network device drivers when running paravirtualized guests.

The situation is blurrier when running HVM guests. When you first install a guest Operating System within a HVM guest, it uses the OS's native device drivers that were designed for use with real physical hardware, and Xen and dom0 emulate those devices for the new guest. However, when you then install paravirtual drivers within the guest (these are the "tools" that you install in the guest on XenServer, or XenClient, and likely also on VMware, etc.) - well, then you're in a different configuration again. What you have there is a HVM guest, running a non-paravirtualized OS, but with paravirtual split device drivers.

So, to answer your question, when you're running in fully virtualized mode, you may or may not be using split device drivers -- it depends on whether or not they are actually installed to be used by the guest OS. Recent Linux kernels already include paravirtual drivers that can be active within a HVM domain.

As I understand it, they're closely related, though not exactly the same. Split drivers means that a driver in domU works by communicating with a corresponding driver in dom0. The communication is done via hypercalls that ask the Xen hypervisor to move data between domains. Paravirtualization means that a guest domain knows it's running under a hypervisor and talks to the hypervisor instead of trying to talk to real hardware, so a split driver is a paravirtualized driver, but paravirtualization is a broader concept.

Split drivers aren't used in an HVM domain because the guest OS uses its own normal drivers, which think they're talking to real hardware.