block: implement BDRV_REQ_PREFETCH

[qemu/kevin.git] / qemu-deprecated.texi

@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.