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