docs/system/arm/virt.rst - qemu - Git at Google

 'virt' generic virtual platform (``virt``)
 ==========================================

 The ``virt`` board is a platform which does not correspond to any
 real hardware; it is designed for use in virtual machines.
 It is the recommended board type if you simply want to run
 a guest such as Linux and do not care about reproducing the
 idiosyncrasies and limitations of a particular bit of real-world
 hardware.

 This is a "versioned" board model, so as well as the ``virt`` machine
 type itself (which may have improvements, bugfixes and other minor
 changes between QEMU versions) a version is provided that guarantees
 to have the same behaviour as that of previous QEMU releases, so
 that VM migration will work between QEMU versions. For instance the
 ``virt-5.0`` machine type will behave like the ``virt`` machine from
 the QEMU 5.0 release, and migration should work between ``virt-5.0``
 of the 5.0 release and ``virt-5.0`` of the 5.1 release. Migration
 is not guaranteed to work between different QEMU releases for
 the non-versioned ``virt`` machine type.

 Supported devices
 """""""""""""""""

 The virt board supports:

 - PCI/PCIe devices
 - Flash memory
 - One PL011 UART
 - An RTC
 - The fw_cfg device that allows a guest to obtain data from QEMU
 - A PL061 GPIO controller
 - An optional SMMUv3 IOMMU
 - hotpluggable DIMMs
 - hotpluggable NVDIMMs
 - An MSI controller (GICv2M or ITS). GICv2M is selected by default along
   with GICv2. ITS is selected by default with GICv3 (>= virt-2.7). Note
   that ITS is not modeled in TCG mode.
 - 32 virtio-mmio transport devices
 - running guests using the KVM accelerator on aarch64 hardware
 - large amounts of RAM (at least 255GB, and more if using highmem)
 - many CPUs (up to 512 if using a GICv3 and highmem)
 - Secure-World-only devices if the CPU has TrustZone:

   - A second PL011 UART
   - A second PL061 GPIO controller, with GPIO lines for triggering
     a system reset or system poweroff
   - A secure flash memory
   - 16MB of secure RAM

 Supported guest CPU types:

 - ``cortex-a7`` (32-bit)
 - ``cortex-a15`` (32-bit; the default)
 - ``cortex-a53`` (64-bit)
 - ``cortex-a57`` (64-bit)
 - ``cortex-a72`` (64-bit)
 - ``a64fx`` (64-bit)
 - ``host`` (with KVM only)
 - ``max`` (same as ``host`` for KVM; best possible emulation with TCG)

 Note that the default is ``cortex-a15``, so for an AArch64 guest you must
 specify a CPU type.

 Graphics output is available, but unlike the x86 PC machine types
 there is no default display device enabled: you should select one from
 the Display devices section of "-device help". The recommended option
 is ``virtio-gpu-pci``; this is the only one which will work correctly
 with KVM. You may also need to ensure your guest kernel is configured
 with support for this; see below.

 Machine-specific options
 """"""""""""""""""""""""

 The following machine-specific options are supported:

 secure
   Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the
   Arm Security Extensions (TrustZone). The default is ``off``.

 virtualization
   Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the
   Arm Virtualization Extensions. The default is ``off``.

 mte
   Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the
   Arm Memory Tagging Extensions. The default is ``off``.

 highmem
   Set ``on``/``off`` to enable/disable placing devices and RAM in physical
   address space above 32 bits. The default is ``on`` for machine types
   later than ``virt-2.12``.

 gic-version
   Specify the version of the Generic Interrupt Controller (GIC) to provide.
   Valid values are:

   ``2``
     GICv2. Note that this limits the number of CPUs to 8.
   ``3``
     GICv3. This allows up to 512 CPUs.
   ``4``
     GICv4. Requires ``virtualization`` to be ``on``; allows up to 317 CPUs.
   ``host``
     Use the same GIC version the host provides, when using KVM
   ``max``
     Use the best GIC version possible (same as host when using KVM;
     with TCG this is currently ``3`` if ``virtualization`` is ``off`` and
     ``4`` if ``virtualization`` is ``on``, but this may change in future)

 its
   Set ``on``/``off`` to enable/disable ITS instantiation. The default is ``on``
   for machine types later than ``virt-2.7``.

 iommu
   Set the IOMMU type to create for the guest. Valid values are:

   ``none``
     Don't create an IOMMU (the default)
   ``smmuv3``
     Create an SMMUv3

 ras
   Set ``on``/``off`` to enable/disable reporting host memory errors to a guest
   using ACPI and guest external abort exceptions. The default is off.

 dtb-kaslr-seed
   Set ``on``/``off`` to pass a random seed via the guest dtb
   kaslr-seed node (in both "/chosen" and /secure-chosen) to use
   for features like address space randomisation. The default is
   ``on``. You will want to disable it if your trusted boot chain will
   verify the DTB it is passed. It would be the responsibility of the
   firmware to come up with a seed and pass it on if it wants to.

 Linux guest kernel configuration
 """"""""""""""""""""""""""""""""

 The 'defconfig' for Linux arm and arm64 kernels should include the
 right device drivers for virtio and the PCI controller; however some older
 kernel versions, especially for 32-bit Arm, did not have everything
 enabled by default. If you're not seeing PCI devices that you expect,
 then check that your guest config has::

   CONFIG_PCI=y
   CONFIG_VIRTIO_PCI=y
   CONFIG_PCI_HOST_GENERIC=y

 If you want to use the ``virtio-gpu-pci`` graphics device you will also
 need::

   CONFIG_DRM=y
   CONFIG_DRM_VIRTIO_GPU=y

 Hardware configuration information for bare-metal programming
 """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""

 The ``virt`` board automatically generates a device tree blob ("dtb")
 which it passes to the guest. This provides information about the
 addresses, interrupt lines and other configuration of the various devices
 in the system. Guest code can rely on and hard-code the following
 addresses:

 - Flash memory starts at address 0x0000_0000

 - RAM starts at 0x4000_0000

 All other information about device locations may change between
 QEMU versions, so guest code must look in the DTB.

 QEMU supports two types of guest image boot for ``virt``, and
 the way for the guest code to locate the dtb binary differs:

 - For guests using the Linux kernel boot protocol (this means any
   non-ELF file passed to the QEMU ``-kernel`` option) the address
   of the DTB is passed in a register (``r2`` for 32-bit guests,
   or ``x0`` for 64-bit guests)

 - For guests booting as "bare-metal" (any other kind of boot),
   the DTB is at the start of RAM (0x4000_0000)
	'virt' generic virtual platform (``virt``)
	==========================================

	The ``virt`` board is a platform which does not correspond to any
	real hardware; it is designed for use in virtual machines.
	It is the recommended board type if you simply want to run
	a guest such as Linux and do not care about reproducing the
	idiosyncrasies and limitations of a particular bit of real-world
	hardware.

	This is a "versioned" board model, so as well as the ``virt`` machine
	type itself (which may have improvements, bugfixes and other minor
	changes between QEMU versions) a version is provided that guarantees
	to have the same behaviour as that of previous QEMU releases, so
	that VM migration will work between QEMU versions. For instance the
	``virt-5.0`` machine type will behave like the ``virt`` machine from
	the QEMU 5.0 release, and migration should work between ``virt-5.0``
	of the 5.0 release and ``virt-5.0`` of the 5.1 release. Migration
	is not guaranteed to work between different QEMU releases for
	the non-versioned ``virt`` machine type.

	Supported devices
	"""""""""""""""""

	The virt board supports:

	- PCI/PCIe devices
	- Flash memory
	- One PL011 UART
	- An RTC
	- The fw_cfg device that allows a guest to obtain data from QEMU
	- A PL061 GPIO controller
	- An optional SMMUv3 IOMMU
	- hotpluggable DIMMs
	- hotpluggable NVDIMMs
	- An MSI controller (GICv2M or ITS). GICv2M is selected by default along
	with GICv2. ITS is selected by default with GICv3 (>= virt-2.7). Note
	that ITS is not modeled in TCG mode.
	- 32 virtio-mmio transport devices
	- running guests using the KVM accelerator on aarch64 hardware
	- large amounts of RAM (at least 255GB, and more if using highmem)
	- many CPUs (up to 512 if using a GICv3 and highmem)
	- Secure-World-only devices if the CPU has TrustZone:

	- A second PL011 UART
	- A second PL061 GPIO controller, with GPIO lines for triggering
	a system reset or system poweroff
	- A secure flash memory
	- 16MB of secure RAM

	Supported guest CPU types:

	- ``cortex-a7`` (32-bit)
	- ``cortex-a15`` (32-bit; the default)
	- ``cortex-a53`` (64-bit)
	- ``cortex-a57`` (64-bit)
	- ``cortex-a72`` (64-bit)
	- ``a64fx`` (64-bit)
	- ``host`` (with KVM only)
	- ``max`` (same as ``host`` for KVM; best possible emulation with TCG)

	Note that the default is ``cortex-a15``, so for an AArch64 guest you must
	specify a CPU type.

	Graphics output is available, but unlike the x86 PC machine types
	there is no default display device enabled: you should select one from
	the Display devices section of "-device help". The recommended option
	is ``virtio-gpu-pci``; this is the only one which will work correctly
	with KVM. You may also need to ensure your guest kernel is configured
	with support for this; see below.

	Machine-specific options
	""""""""""""""""""""""""

	The following machine-specific options are supported:

	secure
	Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the
	Arm Security Extensions (TrustZone). The default is ``off``.

	virtualization
	Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the
	Arm Virtualization Extensions. The default is ``off``.

	mte
	Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the
	Arm Memory Tagging Extensions. The default is ``off``.

	highmem
	Set ``on``/``off`` to enable/disable placing devices and RAM in physical
	address space above 32 bits. The default is ``on`` for machine types
	later than ``virt-2.12``.

	gic-version
	Specify the version of the Generic Interrupt Controller (GIC) to provide.
	Valid values are:

	``2``
	GICv2. Note that this limits the number of CPUs to 8.
	``3``
	GICv3. This allows up to 512 CPUs.
	``4``
	GICv4. Requires ``virtualization`` to be ``on``; allows up to 317 CPUs.
	``host``
	Use the same GIC version the host provides, when using KVM
	``max``
	Use the best GIC version possible (same as host when using KVM;
	with TCG this is currently ``3`` if ``virtualization`` is ``off`` and
	``4`` if ``virtualization`` is ``on``, but this may change in future)

	its
	Set ``on``/``off`` to enable/disable ITS instantiation. The default is ``on``
	for machine types later than ``virt-2.7``.

	iommu
	Set the IOMMU type to create for the guest. Valid values are:

	``none``
	Don't create an IOMMU (the default)
	``smmuv3``
	Create an SMMUv3

	ras
	Set ``on``/``off`` to enable/disable reporting host memory errors to a guest
	using ACPI and guest external abort exceptions. The default is off.

	dtb-kaslr-seed
	Set ``on``/``off`` to pass a random seed via the guest dtb
	kaslr-seed node (in both "/chosen" and /secure-chosen) to use
	for features like address space randomisation. The default is
	``on``. You will want to disable it if your trusted boot chain will
	verify the DTB it is passed. It would be the responsibility of the
	firmware to come up with a seed and pass it on if it wants to.

	Linux guest kernel configuration
	""""""""""""""""""""""""""""""""

	The 'defconfig' for Linux arm and arm64 kernels should include the
	right device drivers for virtio and the PCI controller; however some older
	kernel versions, especially for 32-bit Arm, did not have everything
	enabled by default. If you're not seeing PCI devices that you expect,
	then check that your guest config has::

	CONFIG_PCI=y
	CONFIG_VIRTIO_PCI=y
	CONFIG_PCI_HOST_GENERIC=y

	If you want to use the ``virtio-gpu-pci`` graphics device you will also
	need::

	CONFIG_DRM=y
	CONFIG_DRM_VIRTIO_GPU=y

	Hardware configuration information for bare-metal programming
	"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""

	The ``virt`` board automatically generates a device tree blob ("dtb")
	which it passes to the guest. This provides information about the
	addresses, interrupt lines and other configuration of the various devices
	in the system. Guest code can rely on and hard-code the following
	addresses:

	- Flash memory starts at address 0x0000_0000

	- RAM starts at 0x4000_0000

	All other information about device locations may change between
	QEMU versions, so guest code must look in the DTB.

	QEMU supports two types of guest image boot for ``virt``, and
	the way for the guest code to locate the dtb binary differs:

	- For guests using the Linux kernel boot protocol (this means any
	non-ELF file passed to the QEMU ``-kernel`` option) the address
	of the DTB is passed in a register (``r2`` for 32-bit guests,
	or ``x0`` for 64-bit guests)

	- For guests booting as "bare-metal" (any other kind of boot),
	the DTB is at the start of RAM (0x4000_0000)