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docs: Describe about virtio-iommu with FDT

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Added a section in "Usage" chapter of "iommu.md" to introduce the
special behavior when virtio-iommu is working with FDT on AArch64.

Signed-off-by: Michael Zhao <michael.zhao@arm.com>
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Michael Zhao 2021-09-17 17:04:23 +08:00 committed by Sebastien Boeuf
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@ -121,6 +121,29 @@ lspci
00:04.0 Unassigned class [ffff]: Red Hat, Inc. Virtio RNG
```
### Work with FDT on AArch64
On AArch64 architecture, the virtual IOMMU can still be used even if ACPI is not
enabled. But the effect is different with what the aforementioned test showed.
When ACPI is disabled, virtual IOMMU is supported through Flattened Device Tree
(FDT). In this case, the guest kernel can not tell which device should be
IOMMU-attached and which should not. No matter how many devices you attached to
the virtual IOMMU by setting `iommu=on` option, all the devices on the PCI bus
will be attached to the virtual IOMMU (except the IOMMU itself). Each of the
devices will be added into a IOMMU group.
As a result, the directory content of `/sys/kernel/iommu_groups` would be:
```bash
ls /sys/kernel/iommu_groups/0/devices/
0000:00:02.0
ls /sys/kernel/iommu_groups/1/devices/
0000:00:03.0
ls /sys/kernel/iommu_groups/2/devices/
0000:00:04.0
```
## Faster mappings
By default, the guest memory is mapped with 4k pages and no huge pages, which