182 lines
6.5 KiB
ReStructuredText
182 lines
6.5 KiB
ReStructuredText
'virt' generic virtual platform (``virt``)
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==========================================
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The ``virt`` board is a platform which does not correspond to any
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real hardware; it is designed for use in virtual machines.
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It is the recommended board type if you simply want to run
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a guest such as Linux and do not care about reproducing the
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idiosyncrasies and limitations of a particular bit of real-world
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hardware.
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This is a "versioned" board model, so as well as the ``virt`` machine
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type itself (which may have improvements, bugfixes and other minor
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changes between QEMU versions) a version is provided that guarantees
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to have the same behaviour as that of previous QEMU releases, so
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that VM migration will work between QEMU versions. For instance the
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``virt-5.0`` machine type will behave like the ``virt`` machine from
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the QEMU 5.0 release, and migration should work between ``virt-5.0``
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of the 5.0 release and ``virt-5.0`` of the 5.1 release. Migration
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is not guaranteed to work between different QEMU releases for
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the non-versioned ``virt`` machine type.
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Supported devices
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"""""""""""""""""
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The virt board supports:
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- PCI/PCIe devices
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- Flash memory
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- One PL011 UART
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- An RTC
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- The fw_cfg device that allows a guest to obtain data from QEMU
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- A PL061 GPIO controller
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- An optional SMMUv3 IOMMU
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- hotpluggable DIMMs
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- hotpluggable NVDIMMs
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- An MSI controller (GICv2M or ITS). GICv2M is selected by default along
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with GICv2. ITS is selected by default with GICv3 (>= virt-2.7). Note
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that ITS is not modeled in TCG mode.
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- 32 virtio-mmio transport devices
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- running guests using the KVM accelerator on aarch64 hardware
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- large amounts of RAM (at least 255GB, and more if using highmem)
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- many CPUs (up to 512 if using a GICv3 and highmem)
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- Secure-World-only devices if the CPU has TrustZone:
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- A second PL011 UART
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- A second PL061 GPIO controller, with GPIO lines for triggering
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a system reset or system poweroff
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- A secure flash memory
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- 16MB of secure RAM
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Supported guest CPU types:
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- ``cortex-a7`` (32-bit)
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- ``cortex-a15`` (32-bit; the default)
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- ``cortex-a53`` (64-bit)
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- ``cortex-a57`` (64-bit)
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- ``cortex-a72`` (64-bit)
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- ``cortex-a76`` (64-bit)
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- ``a64fx`` (64-bit)
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- ``host`` (with KVM only)
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- ``neoverse-n1`` (64-bit)
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- ``max`` (same as ``host`` for KVM; best possible emulation with TCG)
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Note that the default is ``cortex-a15``, so for an AArch64 guest you must
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specify a CPU type.
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Graphics output is available, but unlike the x86 PC machine types
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there is no default display device enabled: you should select one from
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the Display devices section of "-device help". The recommended option
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is ``virtio-gpu-pci``; this is the only one which will work correctly
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with KVM. You may also need to ensure your guest kernel is configured
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with support for this; see below.
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Machine-specific options
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""""""""""""""""""""""""
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The following machine-specific options are supported:
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secure
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Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the
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Arm Security Extensions (TrustZone). The default is ``off``.
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virtualization
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Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the
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Arm Virtualization Extensions. The default is ``off``.
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mte
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Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the
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Arm Memory Tagging Extensions. The default is ``off``.
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highmem
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Set ``on``/``off`` to enable/disable placing devices and RAM in physical
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address space above 32 bits. The default is ``on`` for machine types
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later than ``virt-2.12``.
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gic-version
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Specify the version of the Generic Interrupt Controller (GIC) to provide.
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Valid values are:
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``2``
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GICv2. Note that this limits the number of CPUs to 8.
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``3``
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GICv3. This allows up to 512 CPUs.
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``4``
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GICv4. Requires ``virtualization`` to be ``on``; allows up to 317 CPUs.
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``host``
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Use the same GIC version the host provides, when using KVM
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``max``
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Use the best GIC version possible (same as host when using KVM;
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with TCG this is currently ``3`` if ``virtualization`` is ``off`` and
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``4`` if ``virtualization`` is ``on``, but this may change in future)
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its
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Set ``on``/``off`` to enable/disable ITS instantiation. The default is ``on``
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for machine types later than ``virt-2.7``.
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iommu
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Set the IOMMU type to create for the guest. Valid values are:
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``none``
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Don't create an IOMMU (the default)
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``smmuv3``
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Create an SMMUv3
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ras
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Set ``on``/``off`` to enable/disable reporting host memory errors to a guest
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using ACPI and guest external abort exceptions. The default is off.
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dtb-kaslr-seed
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Set ``on``/``off`` to pass a random seed via the guest dtb
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kaslr-seed node (in both "/chosen" and /secure-chosen) to use
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for features like address space randomisation. The default is
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``on``. You will want to disable it if your trusted boot chain will
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verify the DTB it is passed. It would be the responsibility of the
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firmware to come up with a seed and pass it on if it wants to.
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Linux guest kernel configuration
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""""""""""""""""""""""""""""""""
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The 'defconfig' for Linux arm and arm64 kernels should include the
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right device drivers for virtio and the PCI controller; however some older
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kernel versions, especially for 32-bit Arm, did not have everything
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enabled by default. If you're not seeing PCI devices that you expect,
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then check that your guest config has::
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CONFIG_PCI=y
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CONFIG_VIRTIO_PCI=y
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CONFIG_PCI_HOST_GENERIC=y
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If you want to use the ``virtio-gpu-pci`` graphics device you will also
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need::
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CONFIG_DRM=y
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CONFIG_DRM_VIRTIO_GPU=y
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Hardware configuration information for bare-metal programming
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"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
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The ``virt`` board automatically generates a device tree blob ("dtb")
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which it passes to the guest. This provides information about the
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addresses, interrupt lines and other configuration of the various devices
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in the system. Guest code can rely on and hard-code the following
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addresses:
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- Flash memory starts at address 0x0000_0000
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- RAM starts at 0x4000_0000
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All other information about device locations may change between
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QEMU versions, so guest code must look in the DTB.
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QEMU supports two types of guest image boot for ``virt``, and
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the way for the guest code to locate the dtb binary differs:
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- For guests using the Linux kernel boot protocol (this means any
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non-ELF file passed to the QEMU ``-kernel`` option) the address
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of the DTB is passed in a register (``r2`` for 32-bit guests,
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or ``x0`` for 64-bit guests)
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- For guests booting as "bare-metal" (any other kind of boot),
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the DTB is at the start of RAM (0x4000_0000)
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