QEMU and Kconfig — QEMU documentation

# QEMU and Kconfig

QEMU is a very versatile emulator; it can be built for a variety of targets, where each target can emulate various boards and at the same time different targets can share large amounts of code. For example, a POWER and an x86 board can run the same code to emulate a PCI network card, even though the boards use different PCI host bridges, and they can run the same code to emulate a SCSI disk while using different SCSI adapters. Arm, s390 and x86 boards can all present a virtio-blk disk to their guests, but with three different virtio guest interfaces.

Each QEMU target enables a subset of the boards, devices and buses that are included in QEMU’s source code. As a result, each QEMU executable only links a small subset of the files that form QEMU’s source code; anything that is not needed to support a particular target is culled.

QEMU uses a simple domain-specific language to describe the dependencies between components. This is useful for two reasons:

- new targets and boards can be added without knowing in detail the architecture of the hardware emulation subsystems. Boards only have to list the components they need, and the compiled executable will include all the required dependencies and all the devices that the user can add to that board;

- users can easily build reduced versions of QEMU that support only a subset of boards or devices. For example, by default most targets will include all emulated PCI devices that QEMU supports, but the build process is configurable and it is easy to drop unnecessary (or otherwise unwanted) code to make a leaner binary.

This domain-specific language is based on the Kconfig language that originated in the Linux kernel, though it was heavily simplified and the handling of dependencies is stricter in QEMU.

Unlike Linux, there is no user interface to edit the configuration, which is instead specified in per-target files under the `configs/` directory of the QEMU source tree. This is because, unlike Linux, configuration and dependencies can be treated as a black box when building QEMU; the default configuration that QEMU ships with should be okay in almost all cases.

## The Kconfig language

Kconfig defines configurable components in files named `hw/*/Kconfig`. Note that configurable components are _not_ visible in C code as preprocessor symbols; they are only visible in the Makefile. Each configurable component defines a Makefile variable whose name starts with `CONFIG_`.

All elements have boolean (true/false) type; truth is written as `y`, while falsehood is written `n`. They are defined in a Kconfig stanza like the following:

    config ARM_VIRT
       bool
       imply PCI_DEVICES
       select A15MPCORE
       select ACPI
       select ARM_SMMUV3

The `config` keyword introduces a new configuration element. In the example above, Makefiles will have access to a variable named `CONFIG_ARM_VIRT`, with value `y` or `n` (respectively for boolean true and false).

Boolean expressions can be used within the language, whenever `<expr>` is written in the remainder of this section. The `&&`, `||` and `!` operators respectively denote conjunction (AND), disjunction (OR) and negation (NOT).

The `bool` data type declaration is optional, but it is suggested to include it for clarity and future-proofing. After `bool` the following directives can be included:

dependencies: `depends on <expr>`

> This defines a dependency for this configurable element. Dependencies evaluate an expression and force the value of the variable to false if the expression is false.

reverse dependencies: `select <symbol> [if <expr>]`

> While `depends on` can force a symbol to false, reverse dependencies can be used to force another symbol to true. In the following example, `CONFIG_BAZ` will be true whenever `CONFIG_FOO` is true:

>     config FOO
>       select BAZ

> The optional expression will prevent `select` from having any effect unless it is true.

> Note that unlike Linux’s Kconfig implementation, QEMU will detect contradictions between `depends on` and `select` statements and prevent you from building such a configuration.

default value: `default <value> [if <expr>]`

> Default values are assigned to the config symbol if no other value was set by the user via `configs/*.mak` files, and only if `select` or `depends on` directives do not force the value to true or false respectively. `<value>` can be `y` or `n`; it cannot be an arbitrary Boolean expression. However, a condition for applying the default value can be added with `if`.

> A configuration element can have any number of default values (usually, if more than one default is present, they will have different conditions). If multiple default values satisfy their condition, only the first defined one is active.

reverse default (weak reverse dependency): `imply <symbol> [if <expr>]`

> This is similar to `select` as it applies a lower limit of `y` to another symbol. However, the lower limit is only a default and the “implied” symbol’s value may still be set to `n` from a `configs/*.mak` files. The following two examples are equivalent:

>     config FOO
>       bool
>       imply BAZ

>     config BAZ
>       bool
>       default y if FOO

> The next section explains where to use `imply` or `default y`.

## Guidelines for writing Kconfig files

Configurable elements in QEMU fall under five broad groups. Each group declares its dependencies in different ways:

subsystems, of which buses are a special case

> Example:

>     config SCSI
>       bool

> Subsystems always default to false (they have no `default` directive) and are never visible in `configs/*.mak` files. It’s up to other symbols to `select` whatever subsystems they require.

> They sometimes have `select` directives to bring in other required subsystems or buses. For example, `AUX` (the DisplayPort auxiliary channel “bus”) selects `I2C` because it can act as an I2C master too.

devices

> Example:

>     config MEGASAS_SCSI_PCI
>       bool
>       default y if PCI_DEVICES
>       depends on PCI
>       select SCSI

> Devices are the most complex of the five. They can have a variety of directives that cooperate so that a default configuration includes all the devices that can be accessed from QEMU.

> Devices depend on the bus that they lie on, for example a PCI device would specify `depends on PCI`. An MMIO device will likely have no `depends on` directive. Devices also select the buses that the device provides, for example a SCSI adapter would specify `select SCSI`. Finally, devices are usually `default y` if and only if they have at least one `depends on`; the default could be conditional on a device group.

> Devices also select any optional subsystem that they use; for example a video card might specify `select EDID` if it needs to build EDID information and publish it to the guest.

device groups

> Example:

>     config PCI_DEVICES
>       bool

> Device groups provide a convenient mechanism to enable/disable many devices in one go. This is useful when a set of devices is likely to be enabled/disabled by several targets. Device groups usually need no directive and are not used in the Makefile either; they only appear as conditions for `default y` directives.

> QEMU currently has three device groups, `PCI_DEVICES`, `I2C_DEVICES`, and `TEST_DEVICES`. PCI devices usually have a `default y if PCI_DEVICES` directive rather than just `default y`. This lets some boards (notably s390) easily support a subset of PCI devices, for example only VFIO (passthrough) and virtio-pci devices. `I2C_DEVICES` is similar to `PCI_DEVICES`. It contains i2c devices that users might reasonably want to plug in to an i2c bus on any board (and not ones which are very board-specific or that need to be wired up in a way that can’t be done on the command line). `TEST_DEVICES` instead is used for devices that are rarely used on production virtual machines, but provide useful hooks to test QEMU or KVM.

boards

> Example:

>     config SUN4M
>       bool
>       default y
>       depends on SPARC && !SPARC64
>       imply TCX
>       imply CG3
>       select CS4231
>       select ECCMEMCTL
>       select EMPTY_SLOT
>       select ESCC
>       select ESP
>       select FDC
>       select SLAVIO
>       select LANCE
>       select M48T59
>       select STP2000

> Boards specify their constituent devices using `imply` and `select` directives. A device should be listed under `select` if the board cannot be started at all without it. It should be listed under `imply` if (depending on the QEMU command line) the board may or may not be started without it. Boards default to true, but also have a `depends on` clause to limit them to the appropriate targets. For some targets, not all boards may be supported by hardware virtualization, in which case they also depend on the `TCG` symbol, Other symbols that are commonly used as dependencies for boards include libraries (such as `FDT`) or `TARGET_BIG_ENDIAN` (possibly negated).

> Boards are listed for convenience in the `configs/*.mak` for the target they apply to.

internal elements

> Example:

>     config ECCMEMCTL
>       bool
>       select ECC

> Internal elements group code that is useful in several boards or devices. They are usually enabled with `select` and in turn select other elements; they are never visible in `configs/*.mak` files, and often not even in the Makefile.

## Writing and modifying default configurations

In addition to the Kconfig files under hw/, each target also includes a file called `configs/TARGETNAME-softmmu.mak`. These files initialize some Kconfig variables to non-default values and provide the starting point to turn on devices and subsystems.

A file in `configs/` looks like the following example:

    # Default configuration for alpha-softmmu

    # Uncomment the following lines to disable these optional devices:
    #
    #CONFIG_PCI_DEVICES=n
    #CONFIG_TEST_DEVICES=n

    # Boards:
    #
    CONFIG_DP264=y

The first part, consisting of commented-out `=n` assignments, tells the user which devices or device groups are implied by the boards. The second part, consisting of `=y` assignments, tells the user which boards are supported by the target. The user will typically modify the default configuration by uncommenting lines in the first group, or commenting out lines in the second group.

It is also possible to run QEMU’s configure script with the `--without-default-devices` option. When this is done, everything defaults to `n` unless it is `select`ed or explicitly switched on in the `.mak` files. In other words, `default` and `imply` directives are disabled. When QEMU is built with this option, the user will probably want to change some lines in the first group, for example like this:

    CONFIG_PCI_DEVICES=y
    #CONFIG_TEST_DEVICES=n

and/or pick a subset of the devices in those device groups. Without further modifications to `configs/devices/`, a system emulator built without default devices might not do much more than start an empty machine, and even then only if `--nodefaults` is specified on the command line. Starting a VM without `--nodefaults` is allowed to fail, but should never abort. Failures in `make check` with `--without-default-devices` are considered bugs in the test code: the tests should either use `--nodefaults`, and should be skipped if a necessary device is not present in the build. Such failures should not be worked around with `select` directives.

Right now there is no single place that lists all the optional devices for `CONFIG_PCI_DEVICES` and `CONFIG_TEST_DEVICES`. In the future, we expect that `.mak` files will be automatically generated, so that they will include all these symbols and some help text on what they do.

## `Kconfig.host`

In some special cases, a configurable element depends on host features that are detected by QEMU’s configure or `meson.build` scripts; for example some devices depend on the availability of KVM or on the presence of a library on the host.

These symbols should be listed in `Kconfig.host` like this:

    config TPM
      bool

and also listed as follows in the top-level meson.build’s host_kconfig variable:

    host_kconfig = \
      (have_tpm ? ['CONFIG_TPM=y'] : []) + \
      (host_os == 'linux' ? ['CONFIG_LINUX=y'] : []) + \
      (have_ivshmem ? ['CONFIG_IVSHMEM=y'] : []) + \
      ...