[...]... explaining how to use Xen in the context of particular vendors’ products, there is a huge need for a definitive technical insider’s guide to the Xen hypervisor itself Continuing the “engine” analogy, there are books available for “cars” that integrate Xen, but no manuals on how to fix the xv xvi Foreword “engine.” The publication of this book is therefore of great importance to the Xen community and the industry... guest combines these advantages 1.6 The Xen Philosophy 1.6 15 The Xen Philosophy The rest of this book will discuss the Xen system in detail, but in order to understand the details, it is worth taking the time to understand the broad design of Xen Understanding this, the philosophy of Xen, makes it easier to see why particular design decisions were made, and how all of the parts fit together 1.6.1 Separation... both the Xen hypervisor and products that deliver it to market Ian Pratt Xen Project Lead and Founder of XenSource Preface This book aims to serve as a guide to the Xen hypervisor The interface to paravirtualized guests is described in detail, along with some description of the internals of the hypervisor itself Any book about an open source project will, by nature, be less detailed than the code of the. .. this mode, the CPU could not be preempted It would execute a sequence of normal instructions and then another instruction would return the CPU to the original mode To perform context switches into the kernel, the userspace code would raise an exception, causing an automatic jump to PALCode This would set a flag in a hidden register and then pass control to the kernel The kernel could then call other PALCode... 14 Chapter 1 The State of Virtualization between the memory controller and the CPU, it is possible for the hypervisor to delegate some of the partitioning to the memory controller Using AMD-V, there are two ways in which the hypervisor can handle memory partitioning In fact, two modes are provided The first, Shadow Page Tables, allows the hypervisor to trap whenever the guest OS attempts to modify its... attempts to describe Anyone wishing to fully understand the Xen hypervisor will find no better source of authoritative information than the code itself This book aims to provide a guided tour, indicating features of interest to help visitors find their way around the code As with many travel books, it is to be hoped that readers will find it an informative read whether or not they visit the code Much of the. .. implement discovery of shared pages This is not to say that complete anarchy reigns The Xen hypervisor only implements these basic mechanisms, but guests are required to cooperate if they want to use them; if a device advertises its presence in one part of the XenStore tree, other guests must know to look there if they want to find a device of this type As such, there are a number of conventions that exist,... virtualization in mind There also needs to be a way of providing access to the device to more than one guest Again, Xen provides only a mechanism The grant table interface allows developers to grant access to memory pages to other guests, in much the same way as POSIX shared memory, whereas the XenStore provides a filesystem-like hierarchy (complete with access control) that can be used to implement discovery... indicate that root access is likely to be required Use as a Text In addition to the traditional uses for hypervisors, Xen makes an excellent teaching tool Early versions of Xen only supported paravirtualized guests, and newer ones continue to support these in addition to unmodified guests The architecture exposed by the hypervisor to paravirtualized guests is very similar to x86, but differs in a number of... Technology (IVT or VT ) The idea behind these is to extend the x86 ISA to make up for the shortcomings in the existing instruction set Conceptually, they can be thought of as adding a “ring -1” above ring 0, allowing the OS to stay where it expects to be and catching attempts to access the hardware directly In implementation, more than one ring is added, but the important thing is that there is an extra . the XenStore 141 8.1 TheXenStoreInterface 141 8.2 NavigatingtheXenStore 142 8.3 TheXenStoreDevice 145 8.4 ReadingandWritingaKey 147 8.4.1 TheUserspaceWay 148 8.4.2 FromtheKernel 150 8.5 OtherOperations. products that deliver it to market. Ian Pratt Xen Project Lead and Founder of XenSource Preface This book aims to serve as a guide to the Xen hypervisor. The interface to paravirtualized guests. not scalable. While there are books explaining how to use Xen in the context of particular vendors’ products, there is a huge need for a definitive technical insider’s guide to the Xen hypervisor itself.