[thirdparty/linux.git] / Documentation / virt / paravirt_ops.rst

.. SPDX-License-Identifier: GPL-2.0

============
Paravirt_ops
============

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-ops 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 of code paths in
   them are very performance critical.
Commit	Line	Data
8373bc4f LNK	1	.. SPDX-License-Identifier: GPL-2.0
	2
	3	============
a2e19991 LR	4	Paravirt_ops
	5	============
	6
	7	Linux provides support for different hypervisor virtualization technologies.
	8	Historically different binary kernels would be required in order to support
	9	different hypervisors, this restriction was removed with pv_ops.
	10	Linux pv_ops is a virtualization API which enables support for different
	11	hypervisors. It allows each hypervisor to override critical operations and
	12	allows a single kernel binary to run on all supported execution environments
	13	including native machine -- without any hypervisors.
	14
	15	pv_ops provides a set of function pointers which represent operations
	16	corresponding to low level critical instructions and high level
	17	functionalities in various areas. pv-ops allows for optimizations at run
	18	time by enabling binary patching of the low-ops critical operations
	19	at boot time.
	20
	21	pv_ops operations are classified into three categories:
	22
	23	- simple indirect call
8373bc4f LNK	24	These operations correspond to high level functionality where it is
8373bc4f LNK	25	known that the overhead of indirect call isn't very important.
a2e19991 LR	26
a2e19991 LR	27	- indirect call which allows optimization with binary patch
8373bc4f LNK	28	Usually these operations correspond to low level critical instructions. They
	29	are called frequently and are performance critical. The overhead is
	30	very important.
a2e19991 LR	31
a2e19991 LR	32	- a set of macros for hand written assembly code
8373bc4f LNK	33	Hand written assembly codes (.S files) also need paravirtualization
	34	because they include sensitive instructions or some of code paths in
	35	them are very performance critical.