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