Re: [Qemu-devel] [PATCH] Kconfig: add documentation

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 11 Feb 2019 17:38:29 +0100
Paolo Bonzini <pbonzini@redhat.com> wrote:

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