docs/system/arm/virt: Document user-creatable SMMUv3

The virt machine now supports creating multiple SMMUv3 instances, each
associated with a separate PCIe root complex.

Update the documentation with an example.

Signed-off-by: Shameer Kolothum <skolothumtho@nvidia.com>
[PMM: some minor wording tweaks]

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

docs/system/arm/virt.rst

@@ -37,7 +37,8 @@ The virt board supports:
 - An RTC
 - The fw_cfg device that allows a guest to obtain data from QEMU
 - A PL061 GPIO controller
-- An optional SMMUv3 IOMMU
+- An optional machine-wide SMMUv3 IOMMU
+- User-creatable SMMUv3 devices (see below for example)
 - hotpluggable DIMMs
 - hotpluggable NVDIMMs
 - An MSI controller (GICv2M or ITS). GICv2M is selected by default along
@@ -176,7 +177,7 @@ iommu
   ``none``
     Don't create an IOMMU (the default)
   ``smmuv3``
-    Create an SMMUv3
+    Create a machine-wide SMMUv3.
 
 default-bus-bypass-iommu
   Set ``on``/``off`` to enable/disable `bypass_iommu
@@ -219,6 +220,36 @@ x-oem-table-id
   Set string (up to 8 bytes) to override the default value of field OEM Table ID
   in ACPI table header.
 
+SMMU configuration
+""""""""""""""""""
+
+Machine-wide SMMUv3 IOMMU
+  Setting the machine-specific option ``iommu=smmuv3`` causes QEMU to
+  create a single, machine-wide SMMUv3 instance that applies to all
+  devices in the PCIe topology.
+
+  For information about selectively bypassing devices, refer to
+  ``docs/bypass-iommu.txt``.
+
+User-creatable SMMUv3 devices
+  You can use the ``-device arm-smmuv3`` option to create multiple
+  user-defined SMMUv3 devices, each associated with a separate PCIe
+  root complex. This is only permitted if the machine-wide SMMUv3
+  (``iommu=smmuv3``) option is not used. Each ``arm-smmuv3`` device
+  uses the ``primary-bus`` sub-option to specify which PCIe root
+  complex it is associated with.
+
+  This model is useful when you want to mirror a host configuration where
+  each NUMA node typically has its own SMMU, allowing the VM topology to
+  align more closely with the host’s hardware layout.
+
+  Example::
+
+      -device arm-smmuv3,primary-bus=pcie.0,id=smmuv3.0
+      ...
+      -device pxb-pcie,id=pcie.1,numa_node=1
+      -device arm-smmuv3,primary-bus=pcie.1,id=smmuv3.1
+
 Linux guest kernel configuration
 """"""""""""""""""""""""""""""""