2020-10-29 15:06:09 +03:00
|
|
|
.. _kconfig:
|
|
|
|
|
2019-02-12 15:06:51 +03:00
|
|
|
================
|
|
|
|
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
|
2020-03-10 00:58:18 +03:00
|
|
|
SCSI adapters. Arm, s390 and x86 boards can all present a virtio-blk
|
2019-02-12 15:06:51 +03:00
|
|
|
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
|
2019-07-23 15:08:04 +03:00
|
|
|
``--without-default-devices`` option. When this is done, everything defaults
|
2019-02-12 15:06:51 +03:00
|
|
|
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 = \
|
2019-05-10 11:11:59 +03:00
|
|
|
$@ $*/config-devices.mak.d $< $(MINIKCONF_INPUTS) \
|
2019-02-12 15:06:51 +03:00
|
|
|
CONFIG_KVM=$(CONFIG_KVM) \
|
|
|
|
CONFIG_SPICE=$(CONFIG_SPICE) \
|
|
|
|
CONFIG_TPM=$(CONFIG_TPM) \
|
|
|
|
...
|