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-a35`` (64-bit) 56- ``cortex-a53`` (64-bit) 57- ``cortex-a57`` (64-bit) 58- ``cortex-a72`` (64-bit) 59- ``cortex-a76`` (64-bit) 60- ``a64fx`` (64-bit) 61- ``host`` (with KVM only) 62- ``neoverse-n1`` (64-bit) 63- ``max`` (same as ``host`` for KVM; best possible emulation with TCG) 64 65Note that the default is ``cortex-a15``, so for an AArch64 guest you must 66specify a CPU type. 67 68Graphics output is available, but unlike the x86 PC machine types 69there is no default display device enabled: you should select one from 70the Display devices section of "-device help". The recommended option 71is ``virtio-gpu-pci``; this is the only one which will work correctly 72with KVM. You may also need to ensure your guest kernel is configured 73with support for this; see below. 74 75Machine-specific options 76"""""""""""""""""""""""" 77 78The following machine-specific options are supported: 79 80secure 81 Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the 82 Arm Security Extensions (TrustZone). The default is ``off``. 83 84virtualization 85 Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the 86 Arm Virtualization Extensions. The default is ``off``. 87 88mte 89 Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the 90 Arm Memory Tagging Extensions. The default is ``off``. 91 92highmem 93 Set ``on``/``off`` to enable/disable placing devices and RAM in physical 94 address space above 32 bits. The default is ``on`` for machine types 95 later than ``virt-2.12``. 96 97compact-highmem 98 Set ``on``/``off`` to enable/disable the compact layout for high memory regions. 99 The default is ``on`` for machine types later than ``virt-7.2``. 100 101gic-version 102 Specify the version of the Generic Interrupt Controller (GIC) to provide. 103 Valid values are: 104 105 ``2`` 106 GICv2. Note that this limits the number of CPUs to 8. 107 ``3`` 108 GICv3. This allows up to 512 CPUs. 109 ``4`` 110 GICv4. Requires ``virtualization`` to be ``on``; allows up to 317 CPUs. 111 ``host`` 112 Use the same GIC version the host provides, when using KVM 113 ``max`` 114 Use the best GIC version possible (same as host when using KVM; 115 with TCG this is currently ``3`` if ``virtualization`` is ``off`` and 116 ``4`` if ``virtualization`` is ``on``, but this may change in future) 117 118its 119 Set ``on``/``off`` to enable/disable ITS instantiation. The default is ``on`` 120 for machine types later than ``virt-2.7``. 121 122iommu 123 Set the IOMMU type to create for the guest. Valid values are: 124 125 ``none`` 126 Don't create an IOMMU (the default) 127 ``smmuv3`` 128 Create an SMMUv3 129 130ras 131 Set ``on``/``off`` to enable/disable reporting host memory errors to a guest 132 using ACPI and guest external abort exceptions. The default is off. 133 134dtb-randomness 135 Set ``on``/``off`` to pass random seeds via the guest DTB 136 rng-seed and kaslr-seed nodes (in both "/chosen" and 137 "/secure-chosen") to use for features like the random number 138 generator and address space randomisation. The default is 139 ``on``. You will want to disable it if your trusted boot chain 140 will verify the DTB it is passed, since this option causes the 141 DTB to be non-deterministic. It would be the responsibility of 142 the firmware to come up with a seed and pass it on if it wants to. 143 144dtb-kaslr-seed 145 A deprecated synonym for dtb-randomness. 146 147Linux guest kernel configuration 148"""""""""""""""""""""""""""""""" 149 150The 'defconfig' for Linux arm and arm64 kernels should include the 151right device drivers for virtio and the PCI controller; however some older 152kernel versions, especially for 32-bit Arm, did not have everything 153enabled by default. If you're not seeing PCI devices that you expect, 154then check that your guest config has:: 155 156 CONFIG_PCI=y 157 CONFIG_VIRTIO_PCI=y 158 CONFIG_PCI_HOST_GENERIC=y 159 160If you want to use the ``virtio-gpu-pci`` graphics device you will also 161need:: 162 163 CONFIG_DRM=y 164 CONFIG_DRM_VIRTIO_GPU=y 165 166Hardware configuration information for bare-metal programming 167""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" 168 169The ``virt`` board automatically generates a device tree blob ("dtb") 170which it passes to the guest. This provides information about the 171addresses, interrupt lines and other configuration of the various devices 172in the system. Guest code can rely on and hard-code the following 173addresses: 174 175- Flash memory starts at address 0x0000_0000 176 177- RAM starts at 0x4000_0000 178 179All other information about device locations may change between 180QEMU versions, so guest code must look in the DTB. 181 182QEMU supports two types of guest image boot for ``virt``, and 183the way for the guest code to locate the dtb binary differs: 184 185- For guests using the Linux kernel boot protocol (this means any 186 non-ELF file passed to the QEMU ``-kernel`` option) the address 187 of the DTB is passed in a register (``r2`` for 32-bit guests, 188 or ``x0`` for 64-bit guests) 189 190- For guests booting as "bare-metal" (any other kind of boot), 191 the DTB is at the start of RAM (0x4000_0000) 192