linux-zen-desktop/Documentation/virt/paravirt_ops.rst

.. SPDX-License-Identifier: GPL-2.0

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Paravirt_ops
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Linux provides support for different hypervisor virtualization technologies.
Historically, different binary kernels would be required in order to support
different hypervisors; this restriction was removed with pv_ops.
Linux pv_ops is a virtualization API which enables support for different
hypervisors. It allows each hypervisor to override critical operations and
allows a single kernel binary to run on all supported execution environments
including native machine -- without any hypervisors.

pv_ops provides a set of function pointers which represent operations
corresponding to low-level critical instructions and high-level
functionalities in various areas. pv_ops allows for optimizations at run
time by enabling binary patching of the low-level critical operations
at boot time.

pv_ops operations are classified into three categories:

- simple indirect call
   These operations correspond to high-level functionality where it is
   known that the overhead of indirect call isn't very important.

- indirect call which allows optimization with binary patch
   Usually these operations correspond to low-level critical instructions. They
   are called frequently and are performance critical. The overhead is
   very important.

- a set of macros for hand written assembly code
   Hand written assembly codes (.S files) also need paravirtualization
   because they include sensitive instructions or some code paths in
   them are very performance critical.