1'virt' generic virtual platform (``virt``) 2========================================== 3 4The ``virt`` board is a platform which does not correspond to any 5real hardware; it is designed for use in virtual machines. 6It is the recommended board type if you simply want to run 7a guest such as Linux and do not care about reproducing the 8idiosyncrasies and limitations of a particular bit of real-world 9hardware. 10 11This is a "versioned" board model, so as well as the ``virt`` machine 12type itself (which may have improvements, bugfixes and other minor 13changes between QEMU versions) a version is provided that guarantees 14to have the same behaviour as that of previous QEMU releases, so 15that VM migration will work between QEMU versions. For instance the 16``virt-5.0`` machine type will behave like the ``virt`` machine from 17the QEMU 5.0 release, and migration should work between ``virt-5.0`` 18of the 5.0 release and ``virt-5.0`` of the 5.1 release. Migration 19is not guaranteed to work between different QEMU releases for 20the non-versioned ``virt`` machine type. 21 22Supported devices 23""""""""""""""""" 24 25The virt board supports: 26 27- PCI/PCIe devices 28- Flash memory 29- One PL011 UART 30- An RTC 31- The fw_cfg device that allows a guest to obtain data from QEMU 32- A PL061 GPIO controller 33- An optional SMMUv3 IOMMU 34- hotpluggable DIMMs 35- hotpluggable NVDIMMs 36- An MSI controller (GICv2M or ITS). GICv2M is selected by default along 37 with GICv2. ITS is selected by default with GICv3 (>= virt-2.7). Note 38 that ITS is not modeled in TCG mode. 39- 32 virtio-mmio transport devices 40- running guests using the KVM accelerator on aarch64 hardware 41- large amounts of RAM (at least 255GB, and more if using highmem) 42- many CPUs (up to 512 if using a GICv3 and highmem) 43- Secure-World-only devices if the CPU has TrustZone: 44 45 - A second PL011 UART 46 - A second PL061 GPIO controller, with GPIO lines for triggering 47 a system reset or system poweroff 48 - A secure flash memory 49 - 16MB of secure RAM 50 51Supported guest CPU types: 52 53- ``cortex-a7`` (32-bit) 54- ``cortex-a15`` (32-bit; the default) 55- ``cortex-a53`` (64-bit) 56- ``cortex-a57`` (64-bit) 57- ``cortex-a72`` (64-bit) 58- ``cortex-a76`` (64-bit) 59- ``a64fx`` (64-bit) 60- ``host`` (with KVM only) 61- ``neoverse-n1`` (64-bit) 62- ``max`` (same as ``host`` for KVM; best possible emulation with TCG) 63 64Note that the default is ``cortex-a15``, so for an AArch64 guest you must 65specify a CPU type. 66 67Graphics output is available, but unlike the x86 PC machine types 68there is no default display device enabled: you should select one from 69the Display devices section of "-device help". The recommended option 70is ``virtio-gpu-pci``; this is the only one which will work correctly 71with KVM. You may also need to ensure your guest kernel is configured 72with support for this; see below. 73 74Machine-specific options 75"""""""""""""""""""""""" 76 77The following machine-specific options are supported: 78 79secure 80 Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the 81 Arm Security Extensions (TrustZone). The default is ``off``. 82 83virtualization 84 Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the 85 Arm Virtualization Extensions. The default is ``off``. 86 87mte 88 Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the 89 Arm Memory Tagging Extensions. The default is ``off``. 90 91highmem 92 Set ``on``/``off`` to enable/disable placing devices and RAM in physical 93 address space above 32 bits. The default is ``on`` for machine types 94 later than ``virt-2.12``. 95 96gic-version 97 Specify the version of the Generic Interrupt Controller (GIC) to provide. 98 Valid values are: 99 100 ``2`` 101 GICv2. Note that this limits the number of CPUs to 8. 102 ``3`` 103 GICv3. This allows up to 512 CPUs. 104 ``4`` 105 GICv4. Requires ``virtualization`` to be ``on``; allows up to 317 CPUs. 106 ``host`` 107 Use the same GIC version the host provides, when using KVM 108 ``max`` 109 Use the best GIC version possible (same as host when using KVM; 110 with TCG this is currently ``3`` if ``virtualization`` is ``off`` and 111 ``4`` if ``virtualization`` is ``on``, but this may change in future) 112 113its 114 Set ``on``/``off`` to enable/disable ITS instantiation. The default is ``on`` 115 for machine types later than ``virt-2.7``. 116 117iommu 118 Set the IOMMU type to create for the guest. Valid values are: 119 120 ``none`` 121 Don't create an IOMMU (the default) 122 ``smmuv3`` 123 Create an SMMUv3 124 125ras 126 Set ``on``/``off`` to enable/disable reporting host memory errors to a guest 127 using ACPI and guest external abort exceptions. The default is off. 128 129dtb-randomness 130 Set ``on``/``off`` to pass random seeds via the guest DTB 131 rng-seed and kaslr-seed nodes (in both "/chosen" and 132 "/secure-chosen") to use for features like the random number 133 generator and address space randomisation. The default is 134 ``on``. You will want to disable it if your trusted boot chain 135 will verify the DTB it is passed, since this option causes the 136 DTB to be non-deterministic. It would be the responsibility of 137 the firmware to come up with a seed and pass it on if it wants to. 138 139dtb-kaslr-seed 140 A deprecated synonym for dtb-randomness. 141 142Linux guest kernel configuration 143"""""""""""""""""""""""""""""""" 144 145The 'defconfig' for Linux arm and arm64 kernels should include the 146right device drivers for virtio and the PCI controller; however some older 147kernel versions, especially for 32-bit Arm, did not have everything 148enabled by default. If you're not seeing PCI devices that you expect, 149then check that your guest config has:: 150 151 CONFIG_PCI=y 152 CONFIG_VIRTIO_PCI=y 153 CONFIG_PCI_HOST_GENERIC=y 154 155If you want to use the ``virtio-gpu-pci`` graphics device you will also 156need:: 157 158 CONFIG_DRM=y 159 CONFIG_DRM_VIRTIO_GPU=y 160 161Hardware configuration information for bare-metal programming 162""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" 163 164The ``virt`` board automatically generates a device tree blob ("dtb") 165which it passes to the guest. This provides information about the 166addresses, interrupt lines and other configuration of the various devices 167in the system. Guest code can rely on and hard-code the following 168addresses: 169 170- Flash memory starts at address 0x0000_0000 171 172- RAM starts at 0x4000_0000 173 174All other information about device locations may change between 175QEMU versions, so guest code must look in the DTB. 176 177QEMU supports two types of guest image boot for ``virt``, and 178the way for the guest code to locate the dtb binary differs: 179 180- For guests using the Linux kernel boot protocol (this means any 181 non-ELF file passed to the QEMU ``-kernel`` option) the address 182 of the DTB is passed in a register (``r2`` for 32-bit guests, 183 or ``x0`` for 64-bit guests) 184 185- For guests booting as "bare-metal" (any other kind of boot), 186 the DTB is at the start of RAM (0x4000_0000) 187