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.

-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.

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.

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.

-watchdog (since 6.2)

Use -device instead.

-smp (“parameter=0” SMP configurations) (since 6.2)

Specified CPU topology parameters must be greater than zero.

In the SMP configuration, users should either provide a CPU topology parameter with a reasonable value (greater than zero) or just omit it and QEMU will compute the missing value.

However, historically it was implicitly allowed for users to provide a parameter with zero value, which is meaningless and could also possibly cause unexpected results in the -smp parsing. So support for this kind of configurations (e.g. -smp 8,sockets=0) is deprecated since 6.2 and will be removed in the near future, users have to ensure that all the topology members described with -smp are greater than zero.

Plugin argument passing through arg=<string> (since 6.1)

Passing TCG plugins arguments through arg= is redundant is makes the command-line less readable, especially when the argument itself consist of a name and a value, e.g. -plugin plugin_name,arg="arg_name=arg_value". Therefore, the usage of arg is redundant. Single-word arguments are treated as short-form boolean values, and passed to plugins as arg_name=on. However, short-form booleans are deprecated and full explicit arg_name=on form is preferred.

-drive if=none for the sifive_u OTP device (since 6.2)

Using -drive if=none to configure the OTP device of the sifive_u RISC-V machine is deprecated. Use -drive if=pflash instead.

QEMU Machine Protocol (QMP) commands

blockdev-open-tray, blockdev-close-tray argument device (since 2.8)

Use argument id instead.

eject argument device (since 2.8)

Use argument id instead.

blockdev-change-medium argument device (since 2.8)

Use argument id instead.

block_set_io_throttle argument device (since 2.8)

Use argument id instead.

blockdev-add empty string argument backing (since 2.10)

Use argument value null instead.

block-commit arguments base and top (since 3.1)

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.

query-qmp-schema return value member values (since 6.2)

Member values in return value elements with meta-type enum is deprecated. Use members instead.

drive-backup (since 6.2)

Use blockdev-backup in combination with blockdev-add instead. This change primarily separates the creation/opening process of the backup target with explicit, separate steps. blockdev-backup uses mostly the same arguments as drive-backup, except the format and mode options are removed in favor of using explicit blockdev-create and blockdev-add calls. See Live Block Device Operations for details.

Incorrectly typed device_add arguments (since 6.2)

Due to shortcomings in the internal implementation of device_add, QEMU incorrectly accepts certain invalid arguments: Any object or list arguments are silently ignored. Other argument types are not checked, but an implicit conversion happens, so that e.g. string values can be assigned to integer device properties or vice versa.

This is a bug in QEMU that will be fixed in the future so that previously accepted incorrect commands will return an error. Users should make sure that all arguments passed to device_add are consistent with the documented property types.

query-sgx return value member section-size (since 7.0)

Member section-size in return value elements with meta-type uint64 is deprecated. Use sections instead.

query-sgx-capabilities return value member section-size (since 7.0)

Member section-size in return value elements with meta-type uint64 is deprecated. Use sections instead.

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.

Host Architectures

BE MIPS (since 7.2)

As Debian 10 (“Buster”) moved into LTS the big endian 32 bit version of MIPS moved out of support making it hard to maintain our cross-compilation CI tests of the architecture. As we no longer have CI coverage support may bitrot away before the deprecation process completes. The little endian variants of MIPS (both 32 and 64 bit) are still a supported host architecture.

QEMU API (QAPI) events

MEM_UNPLUG_ERROR (since 6.2)

Use the more generic event DEVICE_UNPLUG_GUEST_ERROR instead.

System emulator machines

Arm virt machine dtb-kaslr-seed property

The dtb-kaslr-seed property on the virt board has been deprecated; use the new name dtb-randomness instead. The new name better reflects the way this property affects all random data within the device tree blob, not just the kaslr-seed node.

pc-i440fx-1.4 up to pc-i440fx-1.7 (since 7.0)

These old machine types are quite neglected nowadays and thus might have various pitfalls with regards to live migration. Use a newer machine type instead.

Backend options

Using non-persistent backing file with pmem=on (since 6.1)

This option is used when memory-backend-file is consumed by emulated NVDIMM device. However enabling memory-backend-file.pmem option, when backing file is (a) not DAX capable or (b) not on a filesystem that support direct mapping of persistent memory, is not safe and may lead to data loss or corruption in case of host crash. Options are:

  • modify VM configuration to set pmem=off to continue using fake NVDIMM (without persistence guaranties) with backing file on non DAX storage

  • move backing file to NVDIMM storage and keep pmem=on (to have NVDIMM with persistence guaranties).

Device options

Emulated device options

-device virtio-blk,scsi=on|off (since 5.0)

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.

-device sga (since 6.2)

The sga device loads an option ROM for x86 targets which enables SeaBIOS to send messages to the serial console. SeaBIOS 1.11.0 onwards contains native support for this feature and thus use of the option ROM approach is obsolete. The native SeaBIOS support can be activated by using -machine graphics=off.

-device nvme-ns,eui64-default=on|off (since 7.1)

In QEMU versions 6.1, 6.2 and 7.0, the nvme-ns generates an EUI-64 identifier that is not globally unique. If an EUI-64 identifier is required, the user must set it explicitly using the nvme-ns device parameter eui64.

-device nvme,use-intel-id=on|off (since 7.1)

The nvme device originally used a PCI Vendor/Device Identifier combination from Intel that was not properly allocated. Since version 5.2, the controller has used a properly allocated identifier. Deprecate the use-intel-id machine compatibility parameter.

Block device options

"backing": "" (since 2.12)

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)

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"}

Backwards compatibility

Runnability guarantee of CPU models (since 4.1)

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.

Tools

virtiofsd

There is a new Rust implementation of virtiofsd at https://gitlab.com/virtio-fs/virtiofsd; since this is now marked stable, new development should be done on that rather than the existing C version in the QEMU tree. The C version will still accept fixes and patches that are already in development for the moment, but will eventually be deleted from this tree. New deployments should use the Rust version, and existing systems should consider moving to it. The command line and feature set is very close and moving should be simple.

QEMU guest agent

--blacklist command line option (since 7.2)

--blacklist has been replaced by --block-rpcs (which is a better wording for what this option does). The short form -b still stays the same and thus is the preferred way for scripts that should run with both, older and future versions of QEMU.

blacklist config file option (since 7.2)

The blacklist config file option has been renamed to block-rpcs (to be in sync with the renaming of the corresponding command line option).