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 default-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
   imply VFIO_AMD_XGBE
   imply VFIO_XGMAC
   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 default-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 default-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 default-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
  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 also default to false; they are enabled by the default-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 default-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 default-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 default-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 selected 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'] : []) + \
  (targetos == 'linux' ? ['CONFIG_LINUX=y'] : []) + \
  (have_ivshmem ? ['CONFIG_IVSHMEM=y'] : []) + \
  ...