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Xen 3.0 Virtualization Interface Guide
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3.2 Pseudo-Physical Memory

Since physical memory is allocated and freed on a page granularity, there is no guarantee that a domain will receive a contiguous stretch of physical memory. However most operating systems do not have good support for operating in a fragmented physical address space. To aid porting such operating systems to run on top of Xen, we make a distinction between machine memory and pseudo-physical memory.

Put simply, machine memory refers to the entire amount of memory installed in the machine, including that reserved by Xen, in use by various domains, or currently unallocated. We consider machine memory to comprise a set of 4kB machine page frames numbered consecutively starting from 0. Machine frame numbers mean the same within Xen or any domain.

Pseudo-physical memory, on the other hand, is a per-domain abstraction. It allows a guest operating system to consider its memory allocation to consist of a contiguous range of physical page frames starting at physical frame 0, despite the fact that the underlying machine page frames may be sparsely allocated and in any order.

To achieve this, Xen maintains a globally readable machine-to-physical table which records the mapping from machine page frames to pseudo-physical ones. In addition, each domain is supplied with a physical-to-machine table which performs the inverse mapping. Clearly the machine-to-physical table has size proportional to the amount of RAM installed in the machine, while each physical-to-machine table has size proportional to the memory allocation of the given domain.

Architecture dependent code in guest operating systems can then use the two tables to provide the abstraction of pseudo-physical memory. In general, only certain specialized parts of the operating system (such as page table management) needs to understand the difference between machine and pseudo-physical addresses.

Xen 3.0 Virtualization Interface Guide
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  Published under the terms of the GNU General Public License Design by Interspire