kconfig: add documentation

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 307 insertions, 0 deletions

diff --git a/docs/devel/index.rst b/docs/devel/index.rst
index cd0fa6c9ba..6b11e49caa 100644
--- a/docs/devel/index.rst
+++ b/docs/devel/index.rst
@@ -13,6 +13,7 @@ Contents:
 .. toctree::
    :maxdepth: 2
 
+   kconfig
    loads-stores
    memory
    migration
diff --git a/docs/devel/kconfig.rst b/docs/devel/kconfig.rst
new file mode 100644
index 0000000000..cce146f87d
--- /dev/null
+++ b/docs/devel/kconfig.rst
@@ -0,0 +1,306 @@
+================
+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 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
+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
+``--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) \
+      ...