docs: Add '-device intel-iommu' entry

The parameters of intel-iommu device are non-trivial to understand.  Add an
entry for it so that people can reference to it when using.

There're actually a few more options there, but I hide them explicitly because
they shouldn't be used by normal QEMU users.

Cc: Chao Yang <chayang@redhat.com>
Cc: Lei Yang <leiyang@redhat.com>
Cc: Jing Zhao <jinzhao@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20210707154114.197580-1-peterx@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

qemu-options.hx

@@ -926,6 +926,39 @@ SRST
 
 ``-device pci-ipmi-bt,bmc=id``
     Like the KCS interface, but defines a BT interface on the PCI bus.
+
+``-device intel-iommu[,option=...]``
+    This is only supported by ``-machine q35``, which will enable Intel VT-d
+    emulation within the guest.  It supports below options:
+
+    ``intremap=on|off`` (default: auto)
+        This enables interrupt remapping feature.  It's required to enable
+        complete x2apic.  Currently it only supports kvm kernel-irqchip modes
+        ``off`` or ``split``, while full kernel-irqchip is not yet supported.
+        The default value is "auto", which will be decided by the mode of
+        kernel-irqchip.
+
+    ``caching-mode=on|off`` (default: off)
+        This enables caching mode for the VT-d emulated device.  When
+        caching-mode is enabled, each guest DMA buffer mapping will generate an
+        IOTLB invalidation from the guest IOMMU driver to the vIOMMU device in
+        a synchronous way.  It is required for ``-device vfio-pci`` to work
+        with the VT-d device, because host assigned devices requires to setup
+        the DMA mapping on the host before guest DMA starts.
+
+    ``device-iotlb=on|off`` (default: off)
+        This enables device-iotlb capability for the emulated VT-d device.  So
+        far virtio/vhost should be the only real user for this parameter,
+        paired with ats=on configured for the device.
+
+    ``aw-bits=39|48`` (default: 39)
+        This decides the address width of IOVA address space.  The address
+        space has 39 bits width for 3-level IOMMU page tables, and 48 bits for
+        4-level IOMMU page tables.
+
+    Please also refer to the wiki page for general scenarios of VT-d
+    emulation in QEMU: https://wiki.qemu.org/Features/VT-d.
+
 ERST
 
 DEF("name", HAS_ARG, QEMU_OPTION_name,