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

docs/system/arm/virt.rst

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.. _arm-virt:

'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.

VM migration is not guaranteed when using -cpu max, as features supported may change between QEMU versions. To ensure your VM can be migrated, it is recommended to use another cpu model instead.

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

The virt board supports:

  • PCI/PCIe devices

  • CXL Fixed memory windows, root bridges and devices.

  • Flash memory

  • Either one or two PL011 UARTs for the NonSecure World

  • An RTC

  • The fw_cfg device that allows a guest to obtain data from QEMU

  • A PL061 GPIO controller

  • An optional machine-wide SMMUv3 IOMMU

  • User-creatable SMMUv3 devices (see below for example)

  • An optional SBSA Generic Watchdog Timer (see below)

  • hotpluggable DIMMs

  • hotpluggable NVDIMMs

  • An MSI controller (GICv2m or ITS).

    • When using GICv3, ITS is selected by default when available on the platform.
    • If using GICv2, a GICv2m is provided by default instead.
    • When ITS is not available on a GICv3 platform, a GICv2m is provided by default.

  • 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

The second NonSecure UART only exists if a backend is configured explicitly (e.g. with a second -serial command line option) and TrustZone emulation is not enabled.

Supported guest CPU types:

  • cortex-a7 (32-bit)
  • cortex-a15 (32-bit; the default)
  • cortex-a35 (64-bit)
  • cortex-a53 (64-bit)
  • cortex-a55 (64-bit)
  • cortex-a57 (64-bit)
  • cortex-a72 (64-bit)
  • cortex-a76 (64-bit)
  • cortex-a710 (64-bit)
  • a64fx (64-bit)
  • host (with KVM and HVF only)
  • neoverse-n1 (64-bit)
  • neoverse-v1 (64-bit)
  • neoverse-n2 (64-bit)
  • max (same as host for KVM and HVF; best possible emulation with TCG)

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

Also, please note that passing max CPU (i.e. -cpu max) won't enable all the CPU features for a given virt machine. Where a CPU architectural feature requires support in both the CPU itself and in the wider system (e.g. the MTE feature), it may not be enabled by default, but instead requires a machine option to enable it.

For example, MTE support must be enabled with -machine virt,mte=on, as well as by selecting an MTE-capable CPU (e.g., max) with the -cpu option.

See the machine-specific options below, or check them for a given machine by passing the help suboption, like: -machine virt-9.0,help.

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 when the CPU supports an address space bigger than 32 bits (i.e. 64-bit CPUs, and 32-bit CPUs with the Large Physical Address Extension (LPAE) feature). If you want to boot a 32-bit kernel which does not have CONFIG_LPAE enabled on a CPU type which implements LPAE, you will need to manually set this to off; otherwise some devices, such as the PCI controller, will not be accessible.

compact-highmem Set on/off to enable/disable the compact layout for high memory regions. The default is on for machine types later than virt-7.2.

highmem-redists Set on/off to enable/disable the high memory region for GICv3 or GICv4 redistributor. The default is on. Setting this to off will limit the maximum number of CPUs when GICv3 or GICv4 is used.

highmem-ecam Set on/off to enable/disable the high memory region for PCI ECAM. The default is on for machine types later than virt-3.0.

highmem-mmio Set on/off to enable/disable the high memory region for PCI MMIO. The default is on.

highmem-mmio-size Set the high memory region size for PCI MMIO. Must be a power of 2 and greater than or equal to the default size (512G).

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. x-5 GICv5 (experimental). This is an experimental emulation of the GICv5, based on the EAC release of the GICv5 architecture specification. Experimental means:

- guest-visible behaviour may change when the final version of
  the specification is released and QEMU implements it
- migration support is not yet implemented
- the GICv5 is not exposed to the guest via ACPI tables, only via DTB
- the way the interrupt controller is exposed to the guest and the
  command line syntax for enabling it may change

The current implementation supports only an EL1 guest (no EL2 or
EL3 and no Realm support), and does not implement the ITS (no
MSI support).

Note that as the GICv5 is an Armv9 feature, enabling it will
automatically disable support for AArch32 at all exception levels
except for EL0 (userspace).

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)

msi Specify the MSI and MSI-X controller (GIC) to provide. Valid values are:

auto Use the best available MSI-X controller option. ITS when supported, GICv2m otherwise. gicv2m GICv2m. Typically used with a GICv2. Also available with a newer GIC. its GICv3 ITS. This is the default option when using a GICv3 or GICv4 with a supported accelerator. off Disable support for MSI/MSI-X interrupts.

its Set on/off to control ITS instantiation. This is a deprecated option, use msi instead.

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

none Don't create an IOMMU (the default) smmuv3 Create a machine-wide SMMUv3.

default-bus-bypass-iommu Set on/off to enable/disable bypass_iommu <https://gitlab.com/qemu-project/qemu/-/blob/master/docs/bypass-iommu.txt>_ for default root bus.

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.

acpi Set on/off/auto to enable/disable ACPI.

cxl Set on/off to enable/disable CXL. More details in :doc:../devices/cxl. The default is off.

cxl-fmw Array of CXL fixed memory windows describing fixed address routing to target CXL host bridges. See :doc:../devices/cxl.

dtb-randomness Set on/off to pass random seeds via the guest DTB rng-seed and kaslr-seed nodes (in both "/chosen" and "/secure-chosen") to use for features like the random number generator and 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, since this option causes the DTB to be non-deterministic. It would be the responsibility of the firmware to come up with a seed and pass it on if it wants to.

dtb-kaslr-seed A deprecated synonym for dtb-randomness.

virtio-mmio-transports Set the number of virtio-mmio transports to create (between 0 and 32; the default is 32). Unused transports are harmless, but you can use this property to avoid exposing them to the guest if you wish.

x-oem-id Set string (up to 6 bytes) to override the default value of field OEMID in ACPI table header.

x-oem-table-id Set string (up to 8 bytes) to override the default value of field OEM Table ID in ACPI table header.

SMMU configuration """"""""""""""""""

Machine-wide SMMUv3 IOMMU Setting the machine-specific option iommu=smmuv3 causes QEMU to create a single, machine-wide SMMUv3 instance that applies to all devices in the PCIe topology.

For information about selectively bypassing devices, refer to docs/bypass-iommu.txt.

User-creatable SMMUv3 devices You can use the -device arm-smmuv3 option to create multiple user-defined SMMUv3 devices, each associated with a separate PCIe root complex. This is only permitted if the machine-wide SMMUv3 (iommu=smmuv3) option is not used. Each arm-smmuv3 device uses the primary-bus sub-option to specify which PCIe root complex it is associated with.

This model is useful when you want to mirror a host configuration where each NUMA node typically has its own SMMU, allowing the VM topology to align more closely with the host’s hardware layout.

Example::

  -device arm-smmuv3,primary-bus=pcie.0,id=smmuv3.0
  ...
  -device pxb-pcie,id=pcie.1,numa_node=1
  -device arm-smmuv3,primary-bus=pcie.1,id=smmuv3.1

Accelerated SMMUv3 (nested translation)

The accel=on option enables hardware-accelerated nested translation for vfio-pci passthrough devices. In this mode the guest SMMU driver programs its own Stage-1 page tables, with the host SMMUv3 handling both Stage-1 (guest) and Stage-2 (host) translations in hardware. The host SMMUv3 must support nested translation. This mode requires the iommufd backend and is only supported when booting with ACPI (not device tree).

When accel=on, QEMU automatically derives the values for the ril, ats, oas, ssidsize and cmdqv sub-options from the host SMMUv3 capabilities unless they are set explicitly.

Example::

  -device arm-smmuv3,primary-bus=pcie.0,id=smmuv3.0,accel=on

Accelerated SMMUv3 command queues (Tegra241 CMDQV)

The cmdqv sub-option enables NVIDIA Tegra241 Command Queue Virtualization (CMDQV) on supported hosts. With CMDQV, each accelerated arm-smmuv3 device gets dedicated hardware command queues and the guest issues SMMU invalidation commands directly to real hardware, bypassing QEMU and improving throughput for workloads that issue many invalidations. Without it, every invalidation command traps into QEMU.

SBSA Generic Watchdog """""""""""""""""""""

The SBSA Generic Watchdog Timer (GWDT) can be added to the virt machine using -device sbsa-gwdt. It is only supported on the virt machine, which wires up statically assigned MMIO regions and IRQs via machine-specific plug handlers.

Two modes are available:

Native mode (default) The watchdog is described via the ACPI GTDT table and FDT, using the system counter frequency. Example::

-device sbsa-gwdt

WDAT mode The watchdog is described via the ACPI WDAT table (no FDT node), using a 1 kHz timer frequency. WDAT and GTDT watchdog entries are mutually exclusive. Example::

-device sbsa-gwdt,wdat=on

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)