docs/devel/kconfig.rst

7 QEMU is a very versatile emulator; it can be built for a variety of
9 time different targets can share large amounts of code.  For example,
13 SCSI adapters.  Arm, s390 and x86 boards can all present a virtio-blk
16 Each QEMU target enables a subset of the boards, devices and buses that
18 only links a small subset of the files that form QEMU's source code;
21 QEMU uses a simple domain-specific language to describe the dependencies
25   architecture of the hardware emulation subsystems.  Boards only have
30 * users can easily build reduced versions of QEMU that support only a subset
31   of boards or devices.  For example, by default most targets will include
36 This domain-specific language is based on the Kconfig language that
38 the handling of dependencies is stricter in QEMU.
41 is instead specified in per-target files under the ``configs/``
42 directory of the QEMU source tree.  This is because, unlike Linux,
48 --------------------
73 is written in the remainder of this section.  The ``&&``, ``||`` and
78 include it for clarity and future-proofing.  After ``bool`` the following
84   evaluate an expression and force the value of the variable to false
112   A configuration element can have any number of default values (usually,
119   This is similar to ``select`` as it applies a lower limit of ``y``
136 ------------------------------------
141 **subsystems**, of which **buses** are a special case
166   Devices are the most complex of the five.  They can have a variety
167   of directives that cooperate so that a default configuration includes
190   devices in one go.  This is useful when a set of devices is likely to
198   some boards (notably s390) easily support a subset of PCI devices,
199   for example only VFIO (passthrough) and virtio-pci devices.
202   board (and not ones which are very board-specific or that need
258 --------------------------------------------
261 a file called ``configs/TARGETNAME-softmmu.mak``.  These files
262 initialize some Kconfig variables to non-default values and provide the
267     # Default configuration for alpha-softmmu
278 The first part, consisting of commented-out ``=n`` assignments, tells
280 The second part, consisting of ``=y`` assignments, tells the user which
286 ``--without-default-devices`` option.  When this is done, everything defaults
295 and/or pick a subset of the devices in those device groups.  Without
298 machine, and even then only if ``--nodefaults`` is specified on the
299 command line.  Starting a VM *without* ``--nodefaults`` is allowed to
301 ``--without-default-devices`` are considered bugs in the test code:
302 the tests should either use ``--nodefaults``, and should be skipped
312 ----------------
316 example some devices depend on the availability of KVM or on the presence
317 of a library on the host.
324 and also listed as follows in the top-level meson.build's host_kconfig