qemu-deprecated.texi - qemu - Git at Google

 @node Deprecated features
 @appendix Deprecated features

 In general features are intended to be supported indefinitely once
 introduced into QEMU. In the event that a feature needs to be removed,
 it will be listed in this appendix. The feature will remain functional
 for 2 releases prior to actual removal. Deprecated features may also
 generate warnings on the console when QEMU starts up, or if activated
 via a monitor command, however, this is not a mandatory requirement.

 Prior to the 2.10.0 release there was no official policy on how
 long features would be deprecated prior to their removal, nor
 any documented list of which features were deprecated. Thus
 any features deprecated prior to 2.10.0 will be treated as if
 they were first deprecated in the 2.10.0 release.

 What follows is a list of all features currently marked as
 deprecated.

 @section System emulator command line arguments

 @subsection -machine enforce-config-section=on|off (since 3.1)

 The @option{enforce-config-section} parameter is replaced by the
 @option{-global migration.send-configuration=@var{on|off}} option.

 @subsection -no-kvm (since 1.3.0)

 The ``-no-kvm'' argument is now a synonym for setting
 ``-machine accel=tcg''.

 @subsection -usbdevice (since 2.10.0)

 The ``-usbdevice DEV'' argument is now a synonym for setting
 the ``-device usb-DEV'' argument instead. The deprecated syntax
 would automatically enable USB support on the machine type.
 If using the new syntax, USB support must be explicitly
 enabled via the ``-machine usb=on'' argument.

 @subsection -drive file=json:@{...@{'driver':'file'@}@} (since 3.0)

 The 'file' driver for drives is no longer appropriate for character or host
 devices and will only accept regular files (S_IFREG). The correct driver
 for these file types is 'host_cdrom' or 'host_device' as appropriate.

 @subsection -net ...,name=@var{name} (since 3.1)

 The @option{name} parameter of the @option{-net} option is a synonym
 for the @option{id} parameter, which should now be used instead.

 @subsection -smp (invalid topologies) (since 3.1)

 CPU topology properties should describe whole machine topology including
 possible CPUs.

 However, historically it was possible to start QEMU with an incorrect topology
 where @math{@var{n} <= @var{sockets} * @var{cores} * @var{threads} < @var{maxcpus}},
 which could lead to an incorrect topology enumeration by the guest.
 Support for invalid topologies will be removed, the user must ensure
 topologies described with -smp include all possible cpus, i.e.
   @math{@var{sockets} * @var{cores} * @var{threads} = @var{maxcpus}}.

 @subsection -vnc acl (since 4.0.0)

 The @code{acl} option to the @code{-vnc} argument has been replaced
 by the @code{tls-authz} and @code{sasl-authz} options.

 @subsection QEMU_AUDIO_ environment variables and -audio-help (since 4.0)

 The ``-audiodev'' argument is now the preferred way to specify audio
 backend settings instead of environment variables.  To ease migration to
 the new format, the ``-audiodev-help'' option can be used to convert
 the current values of the environment variables to ``-audiodev'' options.

 @subsection -mon ...,control=readline,pretty=on|off (since 4.1)

 The @code{pretty=on|off} switch has no effect for HMP monitors, but is
 silently ignored. Using the switch with HMP monitors will become an
 error in the future.

 @subsection -realtime (since 4.1)

 The @code{-realtime mlock=on|off} argument has been replaced by the
 @code{-overcommit mem-lock=on|off} argument.

 @subsection -virtfs_synth (since 4.1)

 The ``-virtfs_synth'' argument is now deprecated. Please use ``-fsdev synth''
 and ``-device virtio-9p-...'' instead.

 @subsection -numa node,mem=@var{size} (since 4.1)

 The parameter @option{mem} of @option{-numa node} is used to assign a part of
 guest RAM to a NUMA node. But when using it, it's impossible to manage specified
 RAM chunk on the host side (like bind it to a host node, setting bind policy, ...),
 so guest end-ups with the fake NUMA configuration with suboptiomal performance.
 However since 2014 there is an alternative way to assign RAM to a NUMA node
 using parameter @option{memdev}, which does the same as @option{mem} and adds
 means to actualy manage node RAM on the host side. Use parameter @option{memdev}
 with @var{memory-backend-ram} backend as an replacement for parameter @option{mem}
 to achieve the same fake NUMA effect or a properly configured
 @var{memory-backend-file} backend to actually benefit from NUMA configuration.
 In future new machine versions will not accept the option but it will still
 work with old machine types. User can check QAPI schema to see if the legacy
 option is supported by looking at MachineInfo::numa-mem-supported property.

 @subsection -numa node (without memory specified) (since 4.1)

 Splitting RAM by default between NUMA nodes has the same issues as @option{mem}
 parameter described above with the difference that the role of the user plays
 QEMU using implicit generic or board specific splitting rule.
 Use @option{memdev} with @var{memory-backend-ram} backend or @option{mem} (if
 it's supported by used machine type) to define mapping explictly instead.

 @subsection -mem-path fallback to RAM (since 4.1)
 Currently if guest RAM allocation from file pointed by @option{mem-path}
 fails, QEMU falls back to allocating from RAM, which might result
 in unpredictable behavior since the backing file specified by the user
 is ignored. In the future, users will be responsible for making sure
 the backing storage specified with @option{-mem-path} can actually provide
 the guest RAM configured with @option{-m} and QEMU will fail to start up if
 RAM allocation is unsuccessful.

 @subsection RISC-V -bios (since 4.1)

 QEMU 4.1 introduced support for the -bios option in QEMU for RISC-V for the
 RISC-V virt machine and sifive_u machine.

 QEMU 4.1 has no changes to the default behaviour to avoid breakages. This
 default will change in a future QEMU release, so please prepare now. All users
 of the virt or sifive_u machine must change their command line usage.

 QEMU 4.1 has three options, please migrate to one of these three:
  1. ``-bios none`` - This is the current default behavior if no -bios option
       is included. QEMU will not automatically load any firmware. It is up
       to the user to load all the images they need.
  2. ``-bios default`` - In a future QEMU release this will become the default
       behaviour if no -bios option is specified. This option will load the
       default OpenSBI firmware automatically. The firmware is included with
       the QEMU release and no user interaction is required. All a user needs
       to do is specify the kernel they want to boot with the -kernel option
  3. ``-bios <file>`` - Tells QEMU to load the specified file as the firmwrae.

 @section QEMU Machine Protocol (QMP) commands

 @subsection block-dirty-bitmap-add "autoload" parameter (since 2.12.0)

 "autoload" parameter is now ignored. All bitmaps are automatically loaded
 from qcow2 images.

 @subsection query-block result field dirty-bitmaps[i].status (since 4.0)

 The ``status'' field of the ``BlockDirtyInfo'' structure, returned by
 the query-block command is deprecated. Two new boolean fields,
 ``recording'' and ``busy'' effectively replace it.

 @subsection query-cpus (since 2.12.0)

 The ``query-cpus'' command is replaced by the ``query-cpus-fast'' command.

 @subsection query-cpus-fast "arch" output member (since 3.0.0)

 The ``arch'' output member of the ``query-cpus-fast'' command is
 replaced by the ``target'' output member.

 @subsection cpu-add (since 4.0)

 Use ``device_add'' for hotplugging vCPUs instead of ``cpu-add''.  See
 documentation of ``query-hotpluggable-cpus'' for additional
 details.

 @subsection query-events (since 4.0)

 The ``query-events'' command has been superseded by the more powerful
 and accurate ``query-qmp-schema'' command.

 @subsection chardev client socket with 'wait' option (since 4.0)

 Character devices creating sockets in client mode should not specify
 the 'wait' field, which is only applicable to sockets in server mode

 @section Human Monitor Protocol (HMP) commands

 @subsection The hub_id parameter of 'hostfwd_add' / 'hostfwd_remove' (since 3.1)

 The @option{[hub_id name]} parameter tuple of the 'hostfwd_add' and
 'hostfwd_remove' HMP commands has been replaced by @option{netdev_id}.

 @subsection cpu-add (since 4.0)

 Use ``device_add'' for hotplugging vCPUs instead of ``cpu-add''.  See
 documentation of ``query-hotpluggable-cpus'' for additional details.

 @subsection acl_show, acl_reset, acl_policy, acl_add, acl_remove (since 4.0.0)

 The ``acl_show'', ``acl_reset'', ``acl_policy'', ``acl_add'', and
 ``acl_remove'' commands are deprecated with no replacement. Authorization
 for VNC should be performed using the pluggable QAuthZ objects.

 @section Guest Emulator ISAs

 @subsection RISC-V ISA privledge specification version 1.09.1 (since 4.1)

 The RISC-V ISA privledge specification version 1.09.1 has been deprecated.
 QEMU supports both the newer version 1.10.0 and the ratified version 1.11.0, these
 should be used instead of the 1.09.1 version.

 @section System emulator CPUS

 @subsection RISC-V ISA CPUs (since 4.1)

 The RISC-V cpus with the ISA version in the CPU name have been depcreated. The
 four CPUs are: ``rv32gcsu-v1.9.1``, ``rv32gcsu-v1.10.0``, ``rv64gcsu-v1.9.1`` and
 ``rv64gcsu-v1.10.0``. Instead the version can be specified via the CPU ``priv_spec``
 option when using the ``rv32`` or ``rv64`` CPUs.

 @subsection RISC-V ISA CPUs (since 4.1)

 The RISC-V no MMU cpus have been depcreated. The two CPUs: ``rv32imacu-nommu`` and
 ``rv64imacu-nommu`` should no longer be used. Instead the MMU status can be specified
 via the CPU ``mmu`` option when using the ``rv32`` or ``rv64`` CPUs.

 @section System emulator devices

 @subsection bluetooth (since 3.1)

 The bluetooth subsystem is unmaintained since many years and likely bitrotten
 quite a bit. It will be removed without replacement unless some users speaks
 up at the @email{qemu-devel@@nongnu.org} mailing list with information about
 their usecases.

 @section System emulator machines

 @subsection pc-0.12, pc-0.13, pc-0.14 and pc-0.15 (since 4.0)

 These machine types are very old and likely can not be used for live migration
 from old QEMU versions anymore. A newer machine type should be used instead.

 @subsection prep (PowerPC) (since 3.1)

 This machine type uses an unmaintained firmware, broken in lots of ways,
 and unable to start post-2004 operating systems. 40p machine type should be
 used instead.

 @subsection spike_v1.9.1 and spike_v1.10 (since 4.1)

 The version specific Spike machines have been deprecated in favour of the
 generic ``spike`` machine. If you need to specify an older version of the RISC-V
 spec you can use the ``-cpu rv64gcsu,priv_spec=v1.9.1`` command line argument.

 @section Device options

 @subsection Block device options

 @subsubsection "backing": "" (since 2.12.0)

 In order to prevent QEMU from automatically opening an image's backing
 chain, use ``"backing": null'' instead.

 @subsubsection rbd keyvalue pair encoded filenames: "" (since 3.1.0)

 Options for ``rbd'' should be specified according to its runtime options,
 like other block drivers.  Legacy parsing of keyvalue pair encoded
 filenames is useful to open images with the old format for backing files;
 These image files should be updated to use the current format.

 Example of legacy encoding:

 @code{json:@{"file.driver":"rbd", "file.filename":"rbd:rbd/name"@}}

 The above, converted to the current supported format:

 @code{json:@{"file.driver":"rbd", "file.pool":"rbd", "file.image":"name"@}}

 @section Related binaries

 @subsection qemu-nbd --partition (since 4.0.0)

 The ``qemu-nbd --partition $digit'' code (also spelled @option{-P})
 can only handle MBR partitions, and has never correctly handled
 logical partitions beyond partition 5.  If you know the offset and
 length of the partition (perhaps by using @code{sfdisk} within the
 guest), you can achieve the effect of exporting just that subset of
 the disk by use of the @option{--image-opts} option with a raw
 blockdev using the @code{offset} and @code{size} parameters layered on
 top of any other existing blockdev. For example, if partition 1 is
 100MiB long starting at 1MiB, the old command:

 @code{qemu-nbd -t -P 1 -f qcow2 file.qcow2}

 can be rewritten as:

 @code{qemu-nbd -t --image-opts driver=raw,offset=1M,size=100M,file.driver=qcow2,file.backing.driver=file,file.backing.filename=file.qcow2}

 Alternatively, the @code{nbdkit} project provides a more powerful
 partition filter on top of its nbd plugin, which can be used to select
 an arbitrary MBR or GPT partition on top of any other full-image NBD
 export.  Using this to rewrite the above example results in:

 @code{qemu-nbd -t -k /tmp/sock -f qcow2 file.qcow2 &}
 @code{nbdkit -f --filter=partition nbd socket=/tmp/sock partition=1}

 Note that if you are exposing the export via /dev/nbd0, it is easier
 to just export the entire image and then mount only /dev/nbd0p1 than
 it is to reinvoke @command{qemu-nbd -c /dev/nbd0} limited to just a
 subset of the image.

 @section Build system

 @subsection Python 2 support (since 4.1.0)

 In the future, QEMU will require Python 3 to be available at
 build time.  Support for Python 2 in scripts shipped with QEMU
 is deprecated.

 @section Backwards compatibility

 @subsection Runnability guarantee of CPU models (since 4.1.0)

 Previous versions of QEMU never changed existing CPU models in
 ways that introduced additional host software or hardware
 requirements to the VM.  This allowed management software to
 safely change the machine type of an existing VM without
 introducing new requirements ("runnability guarantee").  This
 prevented CPU models from being updated to include CPU
 vulnerability mitigations, leaving guests vulnerable in the
 default configuration.

 The CPU model runnability guarantee won't apply anymore to
 existing CPU models.  Management software that needs runnability
 guarantees must resolve the CPU model aliases using te
 ``alias-of'' field returned by the ``query-cpu-definitions'' QMP
 command.
	@node Deprecated features
	@appendix Deprecated features

	In general features are intended to be supported indefinitely once
	introduced into QEMU. In the event that a feature needs to be removed,
	it will be listed in this appendix. The feature will remain functional
	for 2 releases prior to actual removal. Deprecated features may also
	generate warnings on the console when QEMU starts up, or if activated
	via a monitor command, however, this is not a mandatory requirement.

	Prior to the 2.10.0 release there was no official policy on how
	long features would be deprecated prior to their removal, nor
	any documented list of which features were deprecated. Thus
	any features deprecated prior to 2.10.0 will be treated as if
	they were first deprecated in the 2.10.0 release.

	What follows is a list of all features currently marked as
	deprecated.

	@section System emulator command line arguments

	@subsection -machine enforce-config-section=on\|off (since 3.1)

	The @option{enforce-config-section} parameter is replaced by the
	@option{-global migration.send-configuration=@var{on\|off}} option.

	@subsection -no-kvm (since 1.3.0)

	The ``-no-kvm'' argument is now a synonym for setting
	``-machine accel=tcg''.

	@subsection -usbdevice (since 2.10.0)

	The ``-usbdevice DEV'' argument is now a synonym for setting
	the ``-device usb-DEV'' argument instead. The deprecated syntax
	would automatically enable USB support on the machine type.
	If using the new syntax, USB support must be explicitly
	enabled via the ``-machine usb=on'' argument.

	@subsection -drive file=json:@{...@{'driver':'file'@}@} (since 3.0)

	The 'file' driver for drives is no longer appropriate for character or host
	devices and will only accept regular files (S_IFREG). The correct driver
	for these file types is 'host_cdrom' or 'host_device' as appropriate.

	@subsection -net ...,name=@var{name} (since 3.1)

	The @option{name} parameter of the @option{-net} option is a synonym
	for the @option{id} parameter, which should now be used instead.

	@subsection -smp (invalid topologies) (since 3.1)

	CPU topology properties should describe whole machine topology including
	possible CPUs.

	However, historically it was possible to start QEMU with an incorrect topology
	where @math{@var{n} <= @var{sockets} * @var{cores} * @var{threads} < @var{maxcpus}},
	which could lead to an incorrect topology enumeration by the guest.
	Support for invalid topologies will be removed, the user must ensure
	topologies described with -smp include all possible cpus, i.e.
	@math{@var{sockets} * @var{cores} * @var{threads} = @var{maxcpus}}.

	@subsection -vnc acl (since 4.0.0)

	The @code{acl} option to the @code{-vnc} argument has been replaced
	by the @code{tls-authz} and @code{sasl-authz} options.

	@subsection QEMU_AUDIO_ environment variables and -audio-help (since 4.0)

	The ``-audiodev'' argument is now the preferred way to specify audio
	backend settings instead of environment variables. To ease migration to
	the new format, the ``-audiodev-help'' option can be used to convert
	the current values of the environment variables to ``-audiodev'' options.

	@subsection -mon ...,control=readline,pretty=on\|off (since 4.1)

	The @code{pretty=on\|off} switch has no effect for HMP monitors, but is
	silently ignored. Using the switch with HMP monitors will become an
	error in the future.

	@subsection -realtime (since 4.1)

	The @code{-realtime mlock=on\|off} argument has been replaced by the
	@code{-overcommit mem-lock=on\|off} argument.

	@subsection -virtfs_synth (since 4.1)

	The ``-virtfs_synth'' argument is now deprecated. Please use ``-fsdev synth''
	and ``-device virtio-9p-...'' instead.

	@subsection -numa node,mem=@var{size} (since 4.1)

	The parameter @option{mem} of @option{-numa node} is used to assign a part of
	guest RAM to a NUMA node. But when using it, it's impossible to manage specified
	RAM chunk on the host side (like bind it to a host node, setting bind policy, ...),
	so guest end-ups with the fake NUMA configuration with suboptiomal performance.
	However since 2014 there is an alternative way to assign RAM to a NUMA node
	using parameter @option{memdev}, which does the same as @option{mem} and adds
	means to actualy manage node RAM on the host side. Use parameter @option{memdev}
	with @var{memory-backend-ram} backend as an replacement for parameter @option{mem}
	to achieve the same fake NUMA effect or a properly configured
	@var{memory-backend-file} backend to actually benefit from NUMA configuration.
	In future new machine versions will not accept the option but it will still
	work with old machine types. User can check QAPI schema to see if the legacy
	option is supported by looking at MachineInfo::numa-mem-supported property.

	@subsection -numa node (without memory specified) (since 4.1)

	Splitting RAM by default between NUMA nodes has the same issues as @option{mem}
	parameter described above with the difference that the role of the user plays
	QEMU using implicit generic or board specific splitting rule.
	Use @option{memdev} with @var{memory-backend-ram} backend or @option{mem} (if
	it's supported by used machine type) to define mapping explictly instead.

	@subsection -mem-path fallback to RAM (since 4.1)
	Currently if guest RAM allocation from file pointed by @option{mem-path}
	fails, QEMU falls back to allocating from RAM, which might result
	in unpredictable behavior since the backing file specified by the user
	is ignored. In the future, users will be responsible for making sure
	the backing storage specified with @option{-mem-path} can actually provide
	the guest RAM configured with @option{-m} and QEMU will fail to start up if
	RAM allocation is unsuccessful.

	@subsection RISC-V -bios (since 4.1)

	QEMU 4.1 introduced support for the -bios option in QEMU for RISC-V for the
	RISC-V virt machine and sifive_u machine.

	QEMU 4.1 has no changes to the default behaviour to avoid breakages. This
	default will change in a future QEMU release, so please prepare now. All users
	of the virt or sifive_u machine must change their command line usage.

	QEMU 4.1 has three options, please migrate to one of these three:
	1. ``-bios none`` - This is the current default behavior if no -bios option
	is included. QEMU will not automatically load any firmware. It is up
	to the user to load all the images they need.
	2. ``-bios default`` - In a future QEMU release this will become the default
	behaviour if no -bios option is specified. This option will load the
	default OpenSBI firmware automatically. The firmware is included with
	the QEMU release and no user interaction is required. All a user needs
	to do is specify the kernel they want to boot with the -kernel option
	3. ``-bios <file>`` - Tells QEMU to load the specified file as the firmwrae.

	@section QEMU Machine Protocol (QMP) commands

	@subsection block-dirty-bitmap-add "autoload" parameter (since 2.12.0)

	"autoload" parameter is now ignored. All bitmaps are automatically loaded
	from qcow2 images.

	@subsection query-block result field dirty-bitmaps[i].status (since 4.0)

	The ``status'' field of the ``BlockDirtyInfo'' structure, returned by
	the query-block command is deprecated. Two new boolean fields,
	``recording'' and ``busy'' effectively replace it.

	@subsection query-cpus (since 2.12.0)

	The ``query-cpus'' command is replaced by the ``query-cpus-fast'' command.

	@subsection query-cpus-fast "arch" output member (since 3.0.0)

	The ``arch'' output member of the ``query-cpus-fast'' command is
	replaced by the ``target'' output member.

	@subsection cpu-add (since 4.0)

	Use ``device_add'' for hotplugging vCPUs instead of ``cpu-add''. See
	documentation of ``query-hotpluggable-cpus'' for additional
	details.

	@subsection query-events (since 4.0)

	The ``query-events'' command has been superseded by the more powerful
	and accurate ``query-qmp-schema'' command.

	@subsection chardev client socket with 'wait' option (since 4.0)

	Character devices creating sockets in client mode should not specify
	the 'wait' field, which is only applicable to sockets in server mode

	@section Human Monitor Protocol (HMP) commands

	@subsection The hub_id parameter of 'hostfwd_add' / 'hostfwd_remove' (since 3.1)

	The @option{[hub_id name]} parameter tuple of the 'hostfwd_add' and
	'hostfwd_remove' HMP commands has been replaced by @option{netdev_id}.

	@subsection cpu-add (since 4.0)

	Use ``device_add'' for hotplugging vCPUs instead of ``cpu-add''. See
	documentation of ``query-hotpluggable-cpus'' for additional details.

	@subsection acl_show, acl_reset, acl_policy, acl_add, acl_remove (since 4.0.0)

	The ``acl_show'', ``acl_reset'', ``acl_policy'', ``acl_add'', and
	``acl_remove'' commands are deprecated with no replacement. Authorization
	for VNC should be performed using the pluggable QAuthZ objects.

	@section Guest Emulator ISAs

	@subsection RISC-V ISA privledge specification version 1.09.1 (since 4.1)

	The RISC-V ISA privledge specification version 1.09.1 has been deprecated.
	QEMU supports both the newer version 1.10.0 and the ratified version 1.11.0, these
	should be used instead of the 1.09.1 version.

	@section System emulator CPUS

	@subsection RISC-V ISA CPUs (since 4.1)

	The RISC-V cpus with the ISA version in the CPU name have been depcreated. The
	four CPUs are: ``rv32gcsu-v1.9.1``, ``rv32gcsu-v1.10.0``, ``rv64gcsu-v1.9.1`` and
	``rv64gcsu-v1.10.0``. Instead the version can be specified via the CPU ``priv_spec``
	option when using the ``rv32`` or ``rv64`` CPUs.

	@subsection RISC-V ISA CPUs (since 4.1)

	The RISC-V no MMU cpus have been depcreated. The two CPUs: ``rv32imacu-nommu`` and
	``rv64imacu-nommu`` should no longer be used. Instead the MMU status can be specified
	via the CPU ``mmu`` option when using the ``rv32`` or ``rv64`` CPUs.

	@section System emulator devices

	@subsection bluetooth (since 3.1)

	The bluetooth subsystem is unmaintained since many years and likely bitrotten
	quite a bit. It will be removed without replacement unless some users speaks
	up at the @email{qemu-devel@@nongnu.org} mailing list with information about
	their usecases.

	@section System emulator machines

	@subsection pc-0.12, pc-0.13, pc-0.14 and pc-0.15 (since 4.0)

	These machine types are very old and likely can not be used for live migration
	from old QEMU versions anymore. A newer machine type should be used instead.

	@subsection prep (PowerPC) (since 3.1)

	This machine type uses an unmaintained firmware, broken in lots of ways,
	and unable to start post-2004 operating systems. 40p machine type should be
	used instead.

	@subsection spike_v1.9.1 and spike_v1.10 (since 4.1)

	The version specific Spike machines have been deprecated in favour of the
	generic ``spike`` machine. If you need to specify an older version of the RISC-V
	spec you can use the ``-cpu rv64gcsu,priv_spec=v1.9.1`` command line argument.

	@section Device options

	@subsection Block device options

	@subsubsection "backing": "" (since 2.12.0)

	In order to prevent QEMU from automatically opening an image's backing
	chain, use ``"backing": null'' instead.

	@subsubsection rbd keyvalue pair encoded filenames: "" (since 3.1.0)

	Options for ``rbd'' should be specified according to its runtime options,
	like other block drivers. Legacy parsing of keyvalue pair encoded
	filenames is useful to open images with the old format for backing files;
	These image files should be updated to use the current format.

	Example of legacy encoding:

	@code{json:@{"file.driver":"rbd", "file.filename":"rbd:rbd/name"@}}

	The above, converted to the current supported format:

	@code{json:@{"file.driver":"rbd", "file.pool":"rbd", "file.image":"name"@}}

	@section Related binaries

	@subsection qemu-nbd --partition (since 4.0.0)

	The ``qemu-nbd --partition $digit'' code (also spelled @option{-P})
	can only handle MBR partitions, and has never correctly handled
	logical partitions beyond partition 5. If you know the offset and
	length of the partition (perhaps by using @code{sfdisk} within the
	guest), you can achieve the effect of exporting just that subset of
	the disk by use of the @option{--image-opts} option with a raw
	blockdev using the @code{offset} and @code{size} parameters layered on
	top of any other existing blockdev. For example, if partition 1 is
	100MiB long starting at 1MiB, the old command:

	@code{qemu-nbd -t -P 1 -f qcow2 file.qcow2}

	can be rewritten as:

	@code{qemu-nbd -t --image-opts driver=raw,offset=1M,size=100M,file.driver=qcow2,file.backing.driver=file,file.backing.filename=file.qcow2}

	Alternatively, the @code{nbdkit} project provides a more powerful
	partition filter on top of its nbd plugin, which can be used to select
	an arbitrary MBR or GPT partition on top of any other full-image NBD
	export. Using this to rewrite the above example results in:

	@code{qemu-nbd -t -k /tmp/sock -f qcow2 file.qcow2 &}
	@code{nbdkit -f --filter=partition nbd socket=/tmp/sock partition=1}

	Note that if you are exposing the export via /dev/nbd0, it is easier
	to just export the entire image and then mount only /dev/nbd0p1 than
	it is to reinvoke @command{qemu-nbd -c /dev/nbd0} limited to just a
	subset of the image.

	@section Build system

	@subsection Python 2 support (since 4.1.0)

	In the future, QEMU will require Python 3 to be available at
	build time. Support for Python 2 in scripts shipped with QEMU
	is deprecated.

	@section Backwards compatibility

	@subsection Runnability guarantee of CPU models (since 4.1.0)

	Previous versions of QEMU never changed existing CPU models in
	ways that introduced additional host software or hardware
	requirements to the VM. This allowed management software to
	safely change the machine type of an existing VM without
	introducing new requirements ("runnability guarantee"). This
	prevented CPU models from being updated to include CPU
	vulnerability mitigations, leaving guests vulnerable in the
	default configuration.

	The CPU model runnability guarantee won't apply anymore to
	existing CPU models. Management software that needs runnability
	guarantees must resolve the CPU model aliases using te
	``alias-of'' field returned by the ``query-cpu-definitions'' QMP
	command.