Deprecated features — QEMU documentation

# 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

### 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`.

### `-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.

### `-no-hpet` (since 8.0)

The HPET setting has been turned into a machine property. Use `-machine hpet=off` instead.

### `-no-acpi` (since 8.0)

The `-no-acpi` setting has been turned into a machine property. Use `-machine acpi=off` instead.

### `-async-teardown` (since 8.1)

Use `-run-with async-teardown=on` instead.

### `-chroot` (since 8.1)

Use `-run-with chroot=dir` instead.

### `-singlestep` (since 8.1)

The `-singlestep` option has been turned into an accelerator property, and given a name that better reflects what it actually does. Use `-accel tcg,one-insn-per-tb=on` instead.

## User-mode emulator command line arguments

### `-singlestep` (since 8.1)

The `-singlestep` option has been given a name that better reflects what it actually does. For both linux-user and bsd-user, use the new `-one-insn-per-tb` option 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.

### `StatusInfo` member `singlestep` (since 8.1)

The `singlestep` member of the `StatusInfo` returned from the `query-status` command is deprecated. This member has a confusing name and it never did what the documentation claimed or what its name suggests. We do not believe that anybody is actually using the information provided in this member.

The information it reports is whether the TCG JIT is in “one instruction per translated block” mode (which can be set on the command line or via the HMP, but not via QMP). The information remains available via the HMP ‘info jit’ command.

## QEMU Machine Protocol (QMP) events

### `MEM_UNPLUG_ERROR` (since 6.2)

Use the more generic event `DEVICE_UNPLUG_GUEST_ERROR` instead.

### `vcpu` trace events (since 8.1)

The ability to instrument QEMU helper functions with vCPU-aware trace points was removed in 7.0. However QMP still exposed the vcpu parameter. This argument has now been deprecated and the remaining remaining trace points that used it are selected just by name.

## Human Monitor Protocol (HMP) commands

### `singlestep` (since 8.1)

The `singlestep` command has been replaced by the `one-insn-per-tb` command, which has the same behaviour but a less misleading name.

## 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.

### System emulation on 32-bit x86 hosts (since 8.0)

Support for 32-bit x86 host deployments is increasingly uncommon in mainstream OS distributions given the widespread availability of 64-bit x86 hardware. The QEMU project no longer considers 32-bit x86 support for system emulation to be an effective use of its limited resources, and thus intends to discontinue it. Since all recent x86 hardware from the past >10 years is capable of the 64-bit x86 extensions, a corresponding 64-bit OS should be used instead.

## System emulator CPUs

### Nios II CPU (since 8.2)

The Nios II architecture is orphan. The `nios2` guest CPU support is deprecated and will be removed in a future version of QEMU.

## System emulator machines

### Arm `virt` machine `dtb-kaslr-seed` property (since 7.1)

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-2.0` up to `pc-i440fx-2.3` (since 8.2)

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.

### Nios II `10m50-ghrd` and `nios2-generic-nommu` machines (since 8.2)

The Nios II architecture is orphan.

## 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 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.

#### `-device cxl-type3,memdev=xxxx` (since 8.0)

The `cxl-type3` device initially only used a single memory backend. With the addition of volatile memory support, it is now necessary to distinguish between persistent and volatile memory backends. As such, memdev is deprecated in favor of persistent-memdev.

#### `-fsdev proxy` and `-virtfs proxy` (since 8.1)

The 9p `proxy` filesystem backend driver has been deprecated and will be removed (along with its proxy helper daemon) in a future version of QEMU. Please use `-fsdev local` or `-virtfs local` for using the 9p `local` filesystem backend, or alternatively consider deploying virtiofsd instead.

The 9p `proxy` backend was originally developed as an alternative to the 9p `local` backend. The idea was to enhance security by dispatching actual low level filesystem operations from 9p server (QEMU process) over to a separate process (the virtfs-proxy-helper binary). However this alternative never gained momentum. The proxy backend is much slower than the local backend, hasn’t seen any development in years, and showed to be less secure, especially due to the fact that its helper daemon must be run as root, whereas with the local backend QEMU is typically run as unprivileged user and allows to tighten behaviour by mapping permissions et al by using its ‘mapped’ security model option.

Nowadays it would make sense to reimplement the `proxy` backend by using QEMU’s `vhost` feature, which would eliminate the high latency costs under which the 9p `proxy` backend currently suffers. However as of to date nobody has indicated plans for such kind of reimplementation unfortunately.

#### RISC-V ‘any’ CPU type `-cpu any` (since 8.2)

The ‘any’ CPU type was introduced back in 2018 and has been around since the initial RISC-V QEMU port. Its usage has always been unclear: users don’t know what to expect from a CPU called ‘any’, and in fact the CPU does not do anything special that isn’t already done by the default CPUs rv32/rv64.

After the introduction of the ‘max’ CPU type, RISC-V now has a good coverage of generic CPUs: rv32 and rv64 as default CPUs and ‘max’ as a feature complete CPU for both 32 and 64 bit builds. Users are then discouraged to use the ‘any’ CPU type starting in 8.2.

#### RISC-V CPU properties which start with capital ‘Z’ (since 8.2)

All RISC-V CPU properties which start with capital ‘Z’ are being deprecated starting in 8.2. The reason is that they were wrongly added with capital ‘Z’ in the past. CPU properties were later added with lower-case names, which is the format we want to use from now on.

Users which try to use these deprecated properties will receive a warning recommending to switch to their stable counterparts:

- “Zifencei” should be replaced with “zifencei”

- “Zicsr” should be replaced with “zicsr”

- “Zihintntl” should be replaced with “zihintntl”

- “Zihintpause” should be replaced with “zihintpause”

- “Zawrs” should be replaced with “zawrs”

- “Zfa” should be replaced with “zfa”

- “Zfh” should be replaced with “zfh”

- “Zfhmin” should be replaced with “zfhmin”

- “Zve32f” should be replaced with “zve32f”

- “Zve64f” should be replaced with “zve64f”

- “Zve64d” should be replaced with “zve64d”

#### `-device pvrdma` and the rdma subsystem (since 8.2)

The pvrdma device and the whole rdma subsystem are in a bad shape and without active maintenance. The QEMU project intends to remove this device and subsystem from the code base in a future release without replacement unless somebody steps up and improves the situation.

### 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"}

#### `iscsi,password=xxx` (since 8.0)

Specifying the iSCSI password in plain text on the command line using the `password` option is insecure. The `password-secret` option should be used instead, to refer to a `--object secret...` instance that provides a password via a file, or encrypted.

### CPU device properties

#### `pmu-num=n` on RISC-V CPUs (since 8.2)

In order to support more flexible counter configurations this has been replaced by a `pmu-mask` property. If set of counters is continuous then the mask can be calculated with `((2 ^ n) - 1) << 3`. The least significant three bits must be left clear.

## 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.

## 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).

## Migration

### `skipped` MigrationStats field (since 8.1)

`skipped` field in Migration stats has been deprecated. It hasn’t been used for more than 10 years.

### `inc` migrate command option (since 8.2)

Use blockdev-mirror with NBD instead.

As an intermediate step the `inc` functionality can be achieved by setting the `block-incremental` migration parameter to `true`. But this parameter is also deprecated.

### `blk` migrate command option (since 8.2)

Use blockdev-mirror with NBD instead.

As an intermediate step the `blk` functionality can be achieved by setting the `block` migration capability to `true`. But this capability is also deprecated.

### block migration (since 8.2)

Block migration is too inflexible. It needs to migrate all block devices or none.

Please see “QMP invocation for live storage migration with `blockdev-mirror` + NBD” in docs/interop/live-block-operations.rst for a detailed explanation.

### old compression method (since 8.2)

Compression method fails too much. Too many races. We are going to remove it if nobody fixes it. For starters, migration-test compression tests are disabled because they fail randomly. If you need compression, use multifd compression methods.