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+Introduction
+------------
+
+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, 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 config option 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 two device groups, ``PCI_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.
+ ``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
+default the 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
+``--with-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. 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 script; 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 KVM
+ bool
+
+and also listed as follows in the top-level Makefile's ``MINIKCONF_ARGS``
+variable::
+
+ MINIKCONF_ARGS = \
+ $@ $*-config.devices.mak.d $< $(MINIKCONF_INPUTS) \
+ CONFIG_KVM=$(CONFIG_KVM) \
+ CONFIG_SPICE=$(CONFIG_SPICE) \
+ CONFIG_TPM=$(CONFIG_TPM) \
+ ...