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