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-a55`` (64-bit) 58- ``cortex-a57`` (64-bit) 59- ``cortex-a72`` (64-bit) 60- ``cortex-a76`` (64-bit) 61- ``cortex-a710`` (64-bit) 62- ``a64fx`` (64-bit) 63- ``host`` (with KVM only) 64- ``neoverse-n1`` (64-bit) 65- ``neoverse-v1`` (64-bit) 66- ``neoverse-n2`` (64-bit) 67- ``max`` (same as ``host`` for KVM; best possible emulation with TCG) 68 69Note that the default is ``cortex-a15``, so for an AArch64 guest you must 70specify a CPU type. 71 72Also, please note that passing ``max`` CPU (i.e. ``-cpu max``) won't 73enable all the CPU features for a given ``virt`` machine. Where a CPU 74architectural feature requires support in both the CPU itself and in the 75wider system (e.g. the MTE feature), it may not be enabled by default, 76but instead requires a machine option to enable it. 77 78For example, MTE support must be enabled with ``-machine virt,mte=on``, 79as well as by selecting an MTE-capable CPU (e.g., ``max``) with the 80``-cpu`` option. 81 82See the machine-specific options below, or check them for a given machine 83by passing the ``help`` suboption, like: ``-machine virt-9.0,help``. 84 85Graphics output is available, but unlike the x86 PC machine types 86there is no default display device enabled: you should select one from 87the Display devices section of "-device help". The recommended option 88is ``virtio-gpu-pci``; this is the only one which will work correctly 89with KVM. You may also need to ensure your guest kernel is configured 90with support for this; see below. 91 92Machine-specific options 93"""""""""""""""""""""""" 94 95The following machine-specific options are supported: 96 97secure 98 Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the 99 Arm Security Extensions (TrustZone). The default is ``off``. 100 101virtualization 102 Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the 103 Arm Virtualization Extensions. The default is ``off``. 104 105mte 106 Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the 107 Arm Memory Tagging Extensions. The default is ``off``. 108 109highmem 110 Set ``on``/``off`` to enable/disable placing devices and RAM in physical 111 address space above 32 bits. The default is ``on`` for machine types 112 later than ``virt-2.12`` when the CPU supports an address space 113 bigger than 32 bits (i.e. 64-bit CPUs, and 32-bit CPUs with the 114 Large Physical Address Extension (LPAE) feature). If you want to 115 boot a 32-bit kernel which does not have ``CONFIG_LPAE`` enabled on 116 a CPU type which implements LPAE, you will need to manually set 117 this to ``off``; otherwise some devices, such as the PCI controller, 118 will not be accessible. 119 120compact-highmem 121 Set ``on``/``off`` to enable/disable the compact layout for high memory regions. 122 The default is ``on`` for machine types later than ``virt-7.2``. 123 124highmem-redists 125 Set ``on``/``off`` to enable/disable the high memory region for GICv3 or 126 GICv4 redistributor. The default is ``on``. Setting this to ``off`` will 127 limit the maximum number of CPUs when GICv3 or GICv4 is used. 128 129highmem-ecam 130 Set ``on``/``off`` to enable/disable the high memory region for PCI ECAM. 131 The default is ``on`` for machine types later than ``virt-3.0``. 132 133highmem-mmio 134 Set ``on``/``off`` to enable/disable the high memory region for PCI MMIO. 135 The default is ``on``. 136 137gic-version 138 Specify the version of the Generic Interrupt Controller (GIC) to provide. 139 Valid values are: 140 141 ``2`` 142 GICv2. Note that this limits the number of CPUs to 8. 143 ``3`` 144 GICv3. This allows up to 512 CPUs. 145 ``4`` 146 GICv4. Requires ``virtualization`` to be ``on``; allows up to 317 CPUs. 147 ``host`` 148 Use the same GIC version the host provides, when using KVM 149 ``max`` 150 Use the best GIC version possible (same as host when using KVM; 151 with TCG this is currently ``3`` if ``virtualization`` is ``off`` and 152 ``4`` if ``virtualization`` is ``on``, but this may change in future) 153 154its 155 Set ``on``/``off`` to enable/disable ITS instantiation. The default is ``on`` 156 for machine types later than ``virt-2.7``. 157 158iommu 159 Set the IOMMU type to create for the guest. Valid values are: 160 161 ``none`` 162 Don't create an IOMMU (the default) 163 ``smmuv3`` 164 Create an SMMUv3 165 166ras 167 Set ``on``/``off`` to enable/disable reporting host memory errors to a guest 168 using ACPI and guest external abort exceptions. The default is off. 169 170dtb-randomness 171 Set ``on``/``off`` to pass random seeds via the guest DTB 172 rng-seed and kaslr-seed nodes (in both "/chosen" and 173 "/secure-chosen") to use for features like the random number 174 generator and address space randomisation. The default is 175 ``on``. You will want to disable it if your trusted boot chain 176 will verify the DTB it is passed, since this option causes the 177 DTB to be non-deterministic. It would be the responsibility of 178 the firmware to come up with a seed and pass it on if it wants to. 179 180dtb-kaslr-seed 181 A deprecated synonym for dtb-randomness. 182 183Linux guest kernel configuration 184"""""""""""""""""""""""""""""""" 185 186The 'defconfig' for Linux arm and arm64 kernels should include the 187right device drivers for virtio and the PCI controller; however some older 188kernel versions, especially for 32-bit Arm, did not have everything 189enabled by default. If you're not seeing PCI devices that you expect, 190then check that your guest config has:: 191 192 CONFIG_PCI=y 193 CONFIG_VIRTIO_PCI=y 194 CONFIG_PCI_HOST_GENERIC=y 195 196If you want to use the ``virtio-gpu-pci`` graphics device you will also 197need:: 198 199 CONFIG_DRM=y 200 CONFIG_DRM_VIRTIO_GPU=y 201 202Hardware configuration information for bare-metal programming 203""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" 204 205The ``virt`` board automatically generates a device tree blob ("dtb") 206which it passes to the guest. This provides information about the 207addresses, interrupt lines and other configuration of the various devices 208in the system. Guest code can rely on and hard-code the following 209addresses: 210 211- Flash memory starts at address 0x0000_0000 212 213- RAM starts at 0x4000_0000 214 215All other information about device locations may change between 216QEMU versions, so guest code must look in the DTB. 217 218QEMU supports two types of guest image boot for ``virt``, and 219the way for the guest code to locate the dtb binary differs: 220 221- For guests using the Linux kernel boot protocol (this means any 222 non-ELF file passed to the QEMU ``-kernel`` option) the address 223 of the DTB is passed in a register (``r2`` for 32-bit guests, 224 or ``x0`` for 64-bit guests) 225 226- For guests booting as "bare-metal" (any other kind of boot), 227 the DTB is at the start of RAM (0x4000_0000) 228