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3. Extensions
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As of Linux 2.6.22, the KVM ABI has been stabilized: no backward
incompatible change are allowed.  However, there is an extension
facility that allows backward-compatible extensions to the API to be
queried and used.

The extension mechanism is not based on the Linux version number.
Instead, kvm defines extension identifiers and a facility to query
whether a particular extension identifier is available.  If it is, a
set of ioctls is available for application use.