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Re: [Qemu-devel] [PATCH] Kconfig: add documentation
From: |
Cornelia Huck |
Subject: |
Re: [Qemu-devel] [PATCH] Kconfig: add documentation |
Date: |
Mon, 11 Feb 2019 18:17:07 +0100 |
On Mon, 11 Feb 2019 17:38:29 +0100
Paolo Bonzini <address@hidden> wrote:
> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
> docs/devel/kconfig.rst | 284 +++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 284 insertions(+)
> create mode 100644 docs/devel/kconfig.rst
>
> diff --git a/docs/devel/kconfig.rst b/docs/devel/kconfig.rst
> new file mode 100644
> index 0000000000..b653c43b12
> --- /dev/null
> +++ b/docs/devel/kconfig.rst
> @@ -0,0 +1,284 @@
> +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 boards can run
> the
s/x86 boards/x86 board/
> +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 both present a
> virtio-blk
s/can both/can all/
> +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;
Is this a stray semicolon, or is something else supposed to follow?
Both points feature full sentences, so I think they should start with a
capital letter?
> +
> +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.
So this is only about devices and friends, and not about cpu features
etc.?
> +
> +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 components
s/components/component/
> +defines a Makefile variable whose name starts with ``CONFIG_``.
> +
> +All elements have boolean (true/false) type. 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).
> +
> +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.
> +
> + ``<expr>`` can be an arbitrary Boolean expression. The ``&&``, ``||`` and
> ``!``
> + operators are supported, respectively for conjunction (AND), disjunction
> + (OR) and negation (NOT).
That seems to be true for any of the <expr> below as well. Maybe
specify it further up instead?
> +
> +**reverse dependencies**: ``select <symbol> [if <expr>]``
> +
> + While ``depends on`` forces a symbol to false, reverse dependencies can be
s/forces/can force/
> + 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.
> +
> + Optionally, 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 which declare
> +their 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**, for example SERIAL
> +
> + Example::
> +
> + config MEGASAS_SCSI_PCI
> + bool
> + default y if PCI_DEVICES
> + depends on PCI
> + select SCSI
It reads a bit odd that you first pick one example, and then show a
completely different one :)
> +
> + 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, if several targets want to do so. Device groups usually need no
> directive
Maybe replace "if several targets want to do so" with "This is useful
when a set of devices is likely to be enabled/disabled by several
targets." ?
> + 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``, so that some boards (notably s390) can easily support only
> VFIO
s/support only/support only e.g./ ?
> + (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 other boards or
> devices.
s/several other/several/ ?
> + They are usually enabled with ``select`` and in turn select other
> elements; they
> + are never visible in ``default-configs/*.mak`` files.
> +
> +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. Doing so disables all the ``default`` and ``imply`` directives. In
> this
I find that sentence a bit confusing. Does it mean that everything
defaults to 'n' that is not explicitly switched on? Maybe an example
would help...
> +case, 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) \
> + ...
> +
Mostly some nitpicking from me; on the whole, I think that's useful
information.