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 97compact-highmem 98 Set ``on``/``off`` to enable/disable the compact layout for high memory regions. 99 The default is ``on`` for machine types later than ``virt-7.2``. 100 101highmem-redists 102 Set ``on``/``off`` to enable/disable the high memory region for GICv3 or 103 GICv4 redistributor. The default is ``on``. Setting this to ``off`` will 104 limit the maximum number of CPUs when GICv3 or GICv4 is used. 105 106highmem-ecam 107 Set ``on``/``off`` to enable/disable the high memory region for PCI ECAM. 108 The default is ``on`` for machine types later than ``virt-3.0``. 109 110highmem-mmio 111 Set ``on``/``off`` to enable/disable the high memory region for PCI MMIO. 112 The default is ``on``. 113 114gic-version 115 Specify the version of the Generic Interrupt Controller (GIC) to provide. 116 Valid values are: 117 118 ``2`` 119 GICv2. Note that this limits the number of CPUs to 8. 120 ``3`` 121 GICv3. This allows up to 512 CPUs. 122 ``4`` 123 GICv4. Requires ``virtualization`` to be ``on``; allows up to 317 CPUs. 124 ``host`` 125 Use the same GIC version the host provides, when using KVM 126 ``max`` 127 Use the best GIC version possible (same as host when using KVM; 128 with TCG this is currently ``3`` if ``virtualization`` is ``off`` and 129 ``4`` if ``virtualization`` is ``on``, but this may change in future) 130 131its 132 Set ``on``/``off`` to enable/disable ITS instantiation. The default is ``on`` 133 for machine types later than ``virt-2.7``. 134 135iommu 136 Set the IOMMU type to create for the guest. Valid values are: 137 138 ``none`` 139 Don't create an IOMMU (the default) 140 ``smmuv3`` 141 Create an SMMUv3 142 143ras 144 Set ``on``/``off`` to enable/disable reporting host memory errors to a guest 145 using ACPI and guest external abort exceptions. The default is off. 146 147dtb-randomness 148 Set ``on``/``off`` to pass random seeds via the guest DTB 149 rng-seed and kaslr-seed nodes (in both "/chosen" and 150 "/secure-chosen") to use for features like the random number 151 generator and address space randomisation. The default is 152 ``on``. You will want to disable it if your trusted boot chain 153 will verify the DTB it is passed, since this option causes the 154 DTB to be non-deterministic. It would be the responsibility of 155 the firmware to come up with a seed and pass it on if it wants to. 156 157dtb-kaslr-seed 158 A deprecated synonym for dtb-randomness. 159 160Linux guest kernel configuration 161"""""""""""""""""""""""""""""""" 162 163The 'defconfig' for Linux arm and arm64 kernels should include the 164right device drivers for virtio and the PCI controller; however some older 165kernel versions, especially for 32-bit Arm, did not have everything 166enabled by default. If you're not seeing PCI devices that you expect, 167then check that your guest config has:: 168 169 CONFIG_PCI=y 170 CONFIG_VIRTIO_PCI=y 171 CONFIG_PCI_HOST_GENERIC=y 172 173If you want to use the ``virtio-gpu-pci`` graphics device you will also 174need:: 175 176 CONFIG_DRM=y 177 CONFIG_DRM_VIRTIO_GPU=y 178 179Hardware configuration information for bare-metal programming 180""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" 181 182The ``virt`` board automatically generates a device tree blob ("dtb") 183which it passes to the guest. This provides information about the 184addresses, interrupt lines and other configuration of the various devices 185in the system. Guest code can rely on and hard-code the following 186addresses: 187 188- Flash memory starts at address 0x0000_0000 189 190- RAM starts at 0x4000_0000 191 192All other information about device locations may change between 193QEMU versions, so guest code must look in the DTB. 194 195QEMU supports two types of guest image boot for ``virt``, and 196the way for the guest code to locate the dtb binary differs: 197 198- For guests using the Linux kernel boot protocol (this means any 199 non-ELF file passed to the QEMU ``-kernel`` option) the address 200 of the DTB is passed in a register (``r2`` for 32-bit guests, 201 or ``x0`` for 64-bit guests) 202 203- For guests booting as "bare-metal" (any other kind of boot), 204 the DTB is at the start of RAM (0x4000_0000) 205