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 97gic-version 98 Specify the version of the Generic Interrupt Controller (GIC) to provide. 99 Valid values are: 100 101 ``2`` 102 GICv2. Note that this limits the number of CPUs to 8. 103 ``3`` 104 GICv3. This allows up to 512 CPUs. 105 ``4`` 106 GICv4. Requires ``virtualization`` to be ``on``; allows up to 317 CPUs. 107 ``host`` 108 Use the same GIC version the host provides, when using KVM 109 ``max`` 110 Use the best GIC version possible (same as host when using KVM; 111 with TCG this is currently ``3`` if ``virtualization`` is ``off`` and 112 ``4`` if ``virtualization`` is ``on``, but this may change in future) 113 114its 115 Set ``on``/``off`` to enable/disable ITS instantiation. The default is ``on`` 116 for machine types later than ``virt-2.7``. 117 118iommu 119 Set the IOMMU type to create for the guest. Valid values are: 120 121 ``none`` 122 Don't create an IOMMU (the default) 123 ``smmuv3`` 124 Create an SMMUv3 125 126ras 127 Set ``on``/``off`` to enable/disable reporting host memory errors to a guest 128 using ACPI and guest external abort exceptions. The default is off. 129 130dtb-randomness 131 Set ``on``/``off`` to pass random seeds via the guest DTB 132 rng-seed and kaslr-seed nodes (in both "/chosen" and 133 "/secure-chosen") to use for features like the random number 134 generator and address space randomisation. The default is 135 ``on``. You will want to disable it if your trusted boot chain 136 will verify the DTB it is passed, since this option causes the 137 DTB to be non-deterministic. It would be the responsibility of 138 the firmware to come up with a seed and pass it on if it wants to. 139 140dtb-kaslr-seed 141 A deprecated synonym for dtb-randomness. 142 143Linux guest kernel configuration 144"""""""""""""""""""""""""""""""" 145 146The 'defconfig' for Linux arm and arm64 kernels should include the 147right device drivers for virtio and the PCI controller; however some older 148kernel versions, especially for 32-bit Arm, did not have everything 149enabled by default. If you're not seeing PCI devices that you expect, 150then check that your guest config has:: 151 152 CONFIG_PCI=y 153 CONFIG_VIRTIO_PCI=y 154 CONFIG_PCI_HOST_GENERIC=y 155 156If you want to use the ``virtio-gpu-pci`` graphics device you will also 157need:: 158 159 CONFIG_DRM=y 160 CONFIG_DRM_VIRTIO_GPU=y 161 162Hardware configuration information for bare-metal programming 163""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" 164 165The ``virt`` board automatically generates a device tree blob ("dtb") 166which it passes to the guest. This provides information about the 167addresses, interrupt lines and other configuration of the various devices 168in the system. Guest code can rely on and hard-code the following 169addresses: 170 171- Flash memory starts at address 0x0000_0000 172 173- RAM starts at 0x4000_0000 174 175All other information about device locations may change between 176QEMU versions, so guest code must look in the DTB. 177 178QEMU supports two types of guest image boot for ``virt``, and 179the way for the guest code to locate the dtb binary differs: 180 181- For guests using the Linux kernel boot protocol (this means any 182 non-ELF file passed to the QEMU ``-kernel`` option) the address 183 of the DTB is passed in a register (``r2`` for 32-bit guests, 184 or ``x0`` for 64-bit guests) 185 186- For guests booting as "bare-metal" (any other kind of boot), 187 the DTB is at the start of RAM (0x4000_0000) 188