xref: /qemu/docs/system/arm/virt.rst (revision 9972ae31)
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  ``4``
103    GICv4. Requires ``virtualization`` to be ``on``; allows up to 317 CPUs.
104  ``host``
105    Use the same GIC version the host provides, when using KVM
106  ``max``
107    Use the best GIC version possible (same as host when using KVM;
108    with TCG this is currently ``3`` if ``virtualization`` is ``off`` and
109    ``4`` if ``virtualization`` is ``on``, but this may change in future)
110
111its
112  Set ``on``/``off`` to enable/disable ITS instantiation. The default is ``on``
113  for machine types later than ``virt-2.7``.
114
115iommu
116  Set the IOMMU type to create for the guest. Valid values are:
117
118  ``none``
119    Don't create an IOMMU (the default)
120  ``smmuv3``
121    Create an SMMUv3
122
123ras
124  Set ``on``/``off`` to enable/disable reporting host memory errors to a guest
125  using ACPI and guest external abort exceptions. The default is off.
126
127dtb-kaslr-seed
128  Set ``on``/``off`` to pass a random seed via the guest dtb
129  kaslr-seed node (in both "/chosen" and /secure-chosen) to use
130  for features like address space randomisation. The default is
131  ``on``. You will want to disable it if your trusted boot chain will
132  verify the DTB it is passed. It would be the responsibility of the
133  firmware to come up with a seed and pass it on if it wants to.
134
135Linux guest kernel configuration
136""""""""""""""""""""""""""""""""
137
138The 'defconfig' for Linux arm and arm64 kernels should include the
139right device drivers for virtio and the PCI controller; however some older
140kernel versions, especially for 32-bit Arm, did not have everything
141enabled by default. If you're not seeing PCI devices that you expect,
142then check that your guest config has::
143
144  CONFIG_PCI=y
145  CONFIG_VIRTIO_PCI=y
146  CONFIG_PCI_HOST_GENERIC=y
147
148If you want to use the ``virtio-gpu-pci`` graphics device you will also
149need::
150
151  CONFIG_DRM=y
152  CONFIG_DRM_VIRTIO_GPU=y
153
154Hardware configuration information for bare-metal programming
155"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
156
157The ``virt`` board automatically generates a device tree blob ("dtb")
158which it passes to the guest. This provides information about the
159addresses, interrupt lines and other configuration of the various devices
160in the system. Guest code can rely on and hard-code the following
161addresses:
162
163- Flash memory starts at address 0x0000_0000
164
165- RAM starts at 0x4000_0000
166
167All other information about device locations may change between
168QEMU versions, so guest code must look in the DTB.
169
170QEMU supports two types of guest image boot for ``virt``, and
171the way for the guest code to locate the dtb binary differs:
172
173- For guests using the Linux kernel boot protocol (this means any
174  non-ELF file passed to the QEMU ``-kernel`` option) the address
175  of the DTB is passed in a register (``r2`` for 32-bit guests,
176  or ``x0`` for 64-bit guests)
177
178- For guests booting as "bare-metal" (any other kind of boot),
179  the DTB is at the start of RAM (0x4000_0000)
180