confidential guest support: Alter virtio default properties for protected guests

The default behaviour for virtio devices is not to use the platforms normal
DMA paths, but instead to use the fact that it's running in a hypervisor
to directly access guest memory.  That doesn't work if the guest's memory
is protected from hypervisor access, such as with AMD's SEV or POWER's PEF.

So, if a confidential guest mechanism is enabled, then apply the
iommu_platform=on option so it will go through normal DMA mechanisms.
Those will presumably have some way of marking memory as shared with
the hypervisor or hardware so that DMA will work.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>

hw/core/machine.c

@@ -33,6 +33,8 @@
 #include "migration/global_state.h"
 #include "migration/vmstate.h"
 #include "exec/confidential-guest-support.h"
+#include "hw/virtio/virtio.h"
+#include "hw/virtio/virtio-pci.h"
 
 GlobalProperty hw_compat_5_2[] = {};
 const size_t hw_compat_5_2_len = G_N_ELEMENTS(hw_compat_5_2);
@@ -1196,6 +1198,17 @@ void machine_run_board_init(MachineState *machine)
          * areas.
          */
         machine_set_mem_merge(OBJECT(machine), false, &error_abort);
+
+        /*
+         * Virtio devices can't count on directly accessing guest
+         * memory, so they need iommu_platform=on to use normal DMA
+         * mechanisms.  That requires also disabling legacy virtio
+         * support for those virtio pci devices which allow it.
+         */
+        object_register_sugar_prop(TYPE_VIRTIO_PCI, "disable-legacy",
+                                   "on", true);
+        object_register_sugar_prop(TYPE_VIRTIO_DEVICE, "iommu_platform",
+                                   "on", false);
     }
 
     machine_class->init(machine);