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 section. The feature will remain functional for the release in which it was deprecated and one further release. After these two releases, the feature is liable to be removed. 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.

System emulator command line arguments

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.

Creating sound card devices and vnc without audiodev= property (since 4.2)

When not using the deprecated legacy audio config, each sound card should specify an audiodev= property. Additionally, when using vnc, you should specify an audiodev= property if you plan to transmit audio through the VNC protocol.

Creating sound card devices using -soundhw (since 5.1)

Sound card devices should be created using -device instead. The names are the same for most devices. The exceptions are hda which needs two devices (-device intel-hda -device hda-duplex) and pcspk which can be activated using -machine pcspk-audiodev=<name>.

-chardev backend aliases tty and parport (since 6.0)

tty and parport are aliases that will be removed. Instead, the actual backend names serial and parallel should be used.

RISC-V -bios (since 5.1)

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 had no changes to the default behaviour to avoid breakages.

QEMU 5.1 changes the default behaviour from -bios none to -bios default.

QEMU 5.1 has three options:

1. -bios default - This is the current default behavior if no -bios option

   is included. 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

2. -bios none - QEMU will not automatically load any firmware. It is up

   to the user to load all the images they need.

3. -bios <file> - Tells QEMU to load the specified file as the firmwrae.

Short-form boolean options (since 6.0)

Boolean options such as share=on/share=off could be written in short form as share and noshare. This is now deprecated and will cause a warning.

delay option for socket character devices (since 6.0)

The replacement for the nodelay short-form boolean option is nodelay=on rather than delay=off.

--enable-fips (since 6.0)

This option restricts usage of certain cryptographic algorithms when the host is operating in FIPS mode.

If FIPS compliance is required, QEMU should be built with the libgcrypt library enabled as a cryptography provider.

Neither the nettle library, or the built-in cryptography provider are supported on FIPS enabled hosts.

-writeconfig (since 6.0)

The -writeconfig option is not able to serialize the entire contents of the QEMU command line. It is thus considered a failed experiment and deprecated, with no current replacement.

Userspace local APIC with KVM (x86, since 6.0)

Using -M kernel-irqchip=off with x86 machine types that include a local APIC is deprecated. The split setting is supported, as is using -M kernel-irqchip=off with the ISA PC machine type.

hexadecimal sizes with scaling multipliers (since 6.0)

Input parameters that take a size value should only use a size suffix (such as ‘k’ or ‘M’) when the base is written in decimal, and not when the value is hexadecimal. That is, ‘0x20M’ is deprecated, and should be written either as ‘32M’ or as ‘0x2000000’.

-spice password=string (since 6.0)

This option is insecure because the SPICE password remains visible in the process listing. This is replaced by the new password-secret option which lets the password be securely provided on the command line using a secret object instance.

opened property of rng-* objects (since 6.0.0)

The only effect of specifying opened=on in the command line or QMP object-add is that the device is opened immediately, possibly before all other options have been processed. This will either have no effect (if opened was the last option) or cause errors. The property is therefore useless and should not be specified.

loaded property of secret and secret_keyring objects (since 6.0.0)

The only effect of specifying loaded=on in the command line or QMP object-add is that the secret is loaded immediately, possibly before all other options have been processed. This will either have no effect (if loaded was the last option) or cause options to be effectively ignored as if they were not given. The property is therefore useless and should not be specified.

QEMU Machine Protocol (QMP) commands

blockdev-open-tray, blockdev-close-tray argument device (since 2.8.0)

Use argument id instead.

eject argument device (since 2.8.0)

Use argument id instead.

blockdev-change-medium argument device (since 2.8.0)

Use argument id instead.

block_set_io_throttle argument device (since 2.8.0)

Use argument id instead.

blockdev-add empty string argument backing (since 2.10.0)

Use argument value null instead.

block-commit arguments base and top (since 3.1.0)

Use arguments base-node and top-node instead.

nbd-server-add and nbd-server-remove (since 5.2)

Use the more generic commands block-export-add and block-export-del instead. As part of this deprecation, where nbd-server-add used a single bitmap, the new block-export-add uses a list of bitmaps.

System accelerators

MIPS Trap-and-Emul KVM support (since 6.0)

The MIPS Trap-and-Emul KVM host and guest support has been removed from Linux upstream kernel, declare it deprecated.

System emulator CPUS

moxie CPU (since 5.2.0)

The moxie guest CPU support is deprecated and will be removed in a future version of QEMU. It’s unclear whether anybody is still using CPU emulation in QEMU, and there are no test images available to make sure that the code is still working.

lm32 CPUs (since 5.2.0)

The lm32 guest CPU support is deprecated and will be removed in a future version of QEMU. The only public user of this architecture was the milkymist project, which has been dead for years; there was never an upstream Linux port.

unicore32 CPUs (since 5.2.0)

The unicore32 guest CPU support is deprecated and will be removed in a future version of QEMU. Support for this CPU was removed from the upstream Linux kernel, and there is no available upstream toolchain to build binaries for it.

Icelake-Client CPU Model (since 5.2.0)

Icelake-Client CPU Models are deprecated. Use Icelake-Server CPU Models instead.

MIPS I7200 CPU Model (since 5.2)

The I7200 guest CPU relies on the nanoMIPS ISA, which is deprecated (the ISA has never been upstreamed to a compiler toolchain). Therefore this CPU is also deprecated.

System emulator machines

Raspberry Pi raspi2 and raspi3 machines (since 5.2)

The Raspberry Pi machines come in various models (A, A+, B, B+). To be able to distinguish which model QEMU is implementing, the raspi2 and raspi3 machines have been renamed raspi2b and raspi3b.

Device options

Emulated device options

-device virtio-blk,scsi=on|off (since 5.0.0)

The virtio-blk SCSI passthrough feature is a legacy VIRTIO feature. VIRTIO 1.0 and later do not support it because the virtio-scsi device was introduced for full SCSI support. Use virtio-scsi instead when SCSI passthrough is required.

Note this also applies to -device virtio-blk-pci,scsi=on|off, which is an alias.

Block device options

"backing": "" (since 2.12.0)

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

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:

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

The above, converted to the current supported format:

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

sheepdog driver (since 5.2.0)

The sheepdog block device driver is deprecated. The corresponding upstream server project is no longer actively maintained. Users are recommended to switch to an alternative distributed block device driver such as RBD. The qemu-img convert command can be used to liberate existing data by moving it out of sheepdog volumes into an alternative storage backend.

linux-user mode CPUs

ppc64abi32 CPUs (since 5.2.0)

The ppc64abi32 architecture has a number of issues which regularly trip up our CI testing and is suspected to be quite broken. For that reason the maintainers strongly suspect no one actually uses it.

MIPS I7200 CPU (since 5.2)

The I7200 guest CPU relies on the nanoMIPS ISA, which is deprecated (the ISA has never been upstreamed to a compiler toolchain). Therefore this CPU is also deprecated.

Related binaries

qemu-img amend to adjust backing file (since 5.1)

The use of qemu-img amend to modify the name or format of a qcow2 backing image is deprecated; this functionality was never fully documented or tested, and interferes with other amend operations that need access to the original backing image (such as deciding whether a v3 zero cluster may be left unallocated when converting to a v2 image). Rather, any changes to the backing chain should be performed with qemu-img rebase -u either before or after the remaining changes being performed by amend, as appropriate.

qemu-img backing file without format (since 5.1)

The use of qemu-img create, qemu-img rebase, or qemu-img convert to create or modify an image that depends on a backing file now recommends that an explicit backing format be provided. This is for safety: if QEMU probes a different format than what you thought, the data presented to the guest will be corrupt; similarly, presenting a raw image to a guest allows a potential security exploit if a future probe sees a non-raw image based on guest writes.

To avoid the warning message, or even future refusal to create an unsafe image, you must pass -o backing_fmt= (or the shorthand -F during create) to specify the intended backing format. You may use qemu-img rebase -u to retroactively add a backing format to an existing image. However, be aware that there are already potential security risks to blindly using qemu-img info to probe the format of an untrusted backing image, when deciding what format to add into an existing image.

Backwards compatibility

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 the alias-of field returned by the query-cpu-definitions QMP command.

While those guarantees are kept, the return value of query-cpu-definitions will have existing CPU model aliases point to a version that doesn’t break runnability guarantees (specifically, version 1 of those CPU models). In future QEMU versions, aliases will point to newer CPU model versions depending on the machine type, so management software must resolve CPU model aliases before starting a virtual machine.

Guest Emulator ISAs

nanoMIPS ISA

The nanoMIPS ISA has never been upstreamed to any compiler toolchain. As it is hard to generate binaries for it, declare it deprecated.