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