xref: /openbmc/qemu/docs/system/arm/virt.rst (revision f6e33708)
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- ``cortex-a710`` (64-bit)
62- ``a64fx`` (64-bit)
63- ``host`` (with KVM only)
64- ``neoverse-n1`` (64-bit)
65- ``neoverse-v1`` (64-bit)
66- ``neoverse-n2`` (64-bit)
67- ``max`` (same as ``host`` for KVM; best possible emulation with TCG)
68
69Note that the default is ``cortex-a15``, so for an AArch64 guest you must
70specify a CPU type.
71
72Also, please note that passing ``max`` CPU (i.e. ``-cpu max``) won't
73enable all the CPU features for a given ``virt`` machine. Where a CPU
74architectural feature requires support in both the CPU itself and in the
75wider system (e.g. the MTE feature), it may not be enabled by default,
76but instead requires a machine option to enable it.
77
78For example, MTE support must be enabled with ``-machine virt,mte=on``,
79as well as by selecting an MTE-capable CPU (e.g., ``max``) with the
80``-cpu`` option.
81
82See the machine-specific options below, or check them for a given machine
83by passing the ``help`` suboption, like: ``-machine virt-9.0,help``.
84
85Graphics output is available, but unlike the x86 PC machine types
86there is no default display device enabled: you should select one from
87the Display devices section of "-device help". The recommended option
88is ``virtio-gpu-pci``; this is the only one which will work correctly
89with KVM. You may also need to ensure your guest kernel is configured
90with support for this; see below.
91
92Machine-specific options
93""""""""""""""""""""""""
94
95The following machine-specific options are supported:
96
97secure
98  Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the
99  Arm Security Extensions (TrustZone). The default is ``off``.
100
101virtualization
102  Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the
103  Arm Virtualization Extensions. The default is ``off``.
104
105mte
106  Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the
107  Arm Memory Tagging Extensions. The default is ``off``.
108
109highmem
110  Set ``on``/``off`` to enable/disable placing devices and RAM in physical
111  address space above 32 bits. The default is ``on`` for machine types
112  later than ``virt-2.12`` when the CPU supports an address space
113  bigger than 32 bits (i.e. 64-bit CPUs, and 32-bit CPUs with the
114  Large Physical Address Extension (LPAE) feature). If you want to
115  boot a 32-bit kernel which does not have ``CONFIG_LPAE`` enabled on
116  a CPU type which implements LPAE, you will need to manually set
117  this to ``off``; otherwise some devices, such as the PCI controller,
118  will not be accessible.
119
120compact-highmem
121  Set ``on``/``off`` to enable/disable the compact layout for high memory regions.
122  The default is ``on`` for machine types later than ``virt-7.2``.
123
124highmem-redists
125  Set ``on``/``off`` to enable/disable the high memory region for GICv3 or
126  GICv4 redistributor. The default is ``on``. Setting this to ``off`` will
127  limit the maximum number of CPUs when GICv3 or GICv4 is used.
128
129highmem-ecam
130  Set ``on``/``off`` to enable/disable the high memory region for PCI ECAM.
131  The default is ``on`` for machine types later than ``virt-3.0``.
132
133highmem-mmio
134  Set ``on``/``off`` to enable/disable the high memory region for PCI MMIO.
135  The default is ``on``.
136
137gic-version
138  Specify the version of the Generic Interrupt Controller (GIC) to provide.
139  Valid values are:
140
141  ``2``
142    GICv2. Note that this limits the number of CPUs to 8.
143  ``3``
144    GICv3. This allows up to 512 CPUs.
145  ``4``
146    GICv4. Requires ``virtualization`` to be ``on``; allows up to 317 CPUs.
147  ``host``
148    Use the same GIC version the host provides, when using KVM
149  ``max``
150    Use the best GIC version possible (same as host when using KVM;
151    with TCG this is currently ``3`` if ``virtualization`` is ``off`` and
152    ``4`` if ``virtualization`` is ``on``, but this may change in future)
153
154its
155  Set ``on``/``off`` to enable/disable ITS instantiation. The default is ``on``
156  for machine types later than ``virt-2.7``.
157
158iommu
159  Set the IOMMU type to create for the guest. Valid values are:
160
161  ``none``
162    Don't create an IOMMU (the default)
163  ``smmuv3``
164    Create an SMMUv3
165
166ras
167  Set ``on``/``off`` to enable/disable reporting host memory errors to a guest
168  using ACPI and guest external abort exceptions. The default is off.
169
170dtb-randomness
171  Set ``on``/``off`` to pass random seeds via the guest DTB
172  rng-seed and kaslr-seed nodes (in both "/chosen" and
173  "/secure-chosen") to use for features like the random number
174  generator and address space randomisation. The default is
175  ``on``. You will want to disable it if your trusted boot chain
176  will verify the DTB it is passed, since this option causes the
177  DTB to be non-deterministic. It would be the responsibility of
178  the firmware to come up with a seed and pass it on if it wants to.
179
180dtb-kaslr-seed
181  A deprecated synonym for dtb-randomness.
182
183Linux guest kernel configuration
184""""""""""""""""""""""""""""""""
185
186The 'defconfig' for Linux arm and arm64 kernels should include the
187right device drivers for virtio and the PCI controller; however some older
188kernel versions, especially for 32-bit Arm, did not have everything
189enabled by default. If you're not seeing PCI devices that you expect,
190then check that your guest config has::
191
192  CONFIG_PCI=y
193  CONFIG_VIRTIO_PCI=y
194  CONFIG_PCI_HOST_GENERIC=y
195
196If you want to use the ``virtio-gpu-pci`` graphics device you will also
197need::
198
199  CONFIG_DRM=y
200  CONFIG_DRM_VIRTIO_GPU=y
201
202Hardware configuration information for bare-metal programming
203"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
204
205The ``virt`` board automatically generates a device tree blob ("dtb")
206which it passes to the guest. This provides information about the
207addresses, interrupt lines and other configuration of the various devices
208in the system. Guest code can rely on and hard-code the following
209addresses:
210
211- Flash memory starts at address 0x0000_0000
212
213- RAM starts at 0x4000_0000
214
215All other information about device locations may change between
216QEMU versions, so guest code must look in the DTB.
217
218QEMU supports two types of guest image boot for ``virt``, and
219the way for the guest code to locate the dtb binary differs:
220
221- For guests using the Linux kernel boot protocol (this means any
222  non-ELF file passed to the QEMU ``-kernel`` option) the address
223  of the DTB is passed in a register (``r2`` for 32-bit guests,
224  or ``x0`` for 64-bit guests)
225
226- For guests booting as "bare-metal" (any other kind of boot),
227  the DTB is at the start of RAM (0x4000_0000)
228