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.

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 hard ware emulation subsystems.
• users can easily build reduced versions of QEMU that support only a subset of boards and devices

The domain-specific language is based on the Kconfig language that originated in the Kernel kernel, though it was heavily simplified and the handling of depedencies is stricter in QEMU.

The Kconfig language

Kconfig defines configurable components in files named hw/*/Kconfig. Note that configurable components are not visible in C code, they are only visible in the Makefile. Each configurable component defines a Makefile variable whose names starts with CONFIG_.

config ARM_VIRT
	bool
	imply PCI_DEVICES
	select A15MPCORE
	select ACPI
	select ARM_SUMMUV3

config keyword introduces CONFIG_ARM_VIRT. After bool the following directives can be included:

• dependencies: depends on <expr>
• reverse dependencies: select <symbol> [if <expr>]
• default value: defalut <value> [if <expr>]
• reverse default(weak reverse dependency): imply <symbol> [if <expr>]

example:

config FOO
	select BAZ # CONFIG_BAZ will be true whenever CONFIG_FOO is true
	
	
config FOO
  bool
  imply BAZ

config BAZ
  bool
  default y if FOO
# Above config are equalvalent