backends/iommufd: Introduce the iommufd object

Introduce an iommufd object which allows the interaction
with the host /dev/iommu device.

The /dev/iommu can have been already pre-opened outside of qemu,
in which case the fd can be passed directly along with the
iommufd object:

This allows the iommufd object to be shared accross several
subsystems (VFIO, VDPA, ...). For example, libvirt would open
the /dev/iommu once.

If no fd is passed along with the iommufd object, the /dev/iommu
is opened by the qemu code.

Suggested-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>

qapi/qom.json

@@ -794,6 +794,23 @@
 { 'struct': 'VfioUserServerProperties',
   'data': { 'socket': 'SocketAddress', 'device': 'str' } }
 
+##
+# @IOMMUFDProperties:
+#
+# Properties for iommufd objects.
+#
+# @fd: file descriptor name previously passed via 'getfd' command,
+#     which represents a pre-opened /dev/iommu.  This allows the
+#     iommufd object to be shared accross several subsystems
+#     (VFIO, VDPA, ...), and the file descriptor to be shared
+#     with other process, e.g. DPDK.  (default: QEMU opens
+#     /dev/iommu by itself)
+#
+# Since: 9.0
+##
+{ 'struct': 'IOMMUFDProperties',
+  'data': { '*fd': 'str' } }
+
 ##
 # @RngProperties:
 #
@@ -934,6 +951,7 @@
     'input-barrier',
     { 'name': 'input-linux',
       'if': 'CONFIG_LINUX' },
+    'iommufd',
     'iothread',
     'main-loop',
     { 'name': 'memory-backend-epc',
@@ -1003,6 +1021,7 @@
       'input-barrier':              'InputBarrierProperties',
       'input-linux':                { 'type': 'InputLinuxProperties',
                                       'if': 'CONFIG_LINUX' },
+      'iommufd':                    'IOMMUFDProperties',
       'iothread':                   'IothreadProperties',
       'main-loop':                  'MainLoopProperties',
       'memory-backend-epc':         { 'type': 'MemoryBackendEpcProperties',