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Xen 3.0 Virtualization Interface Guide
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6. Grant tables

Xen's grant tables provide a generic mechanism to memory sharing between domains. This shared memory interface underpins the split device drivers for block and network IO.

Each domain has its own grant table. This is a data structure that is shared with Xen; it allows the domain to tell Xen what kind of permissions other domains have on its pages. Entries in the grant table are identified by grant references. A grant reference is an integer, which indexes into the grant table. It acts as a capability which the grantee can use to perform operations on the granter's memory.

This capability-based system allows shared-memory communications between unprivileged domains. A grant reference also encapsulates the details of a shared page, removing the need for a domain to know the real machine address of a page it is sharing. This makes it possible to share memory correctly with domains running in fully virtualised memory.

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