From: Zhenzhong Duan <zhenzhong.duan@intel.com>

Call pci_device_get_viommu_flags() to get if vIOMMU supports
VIOMMU_FLAG_WANT_NESTING_PARENT.

If yes, create a nesting parent HWPT and add it to the container's hwpt_list,
letting this parent HWPT cover the entire second stage mappings (GPA=>HPA).

This allows a VFIO passthrough device to directly attach to this default HWPT
and then to use the system address space and its listener.

Introduce a vfio_device_get_viommu_flags_want_nesting() helper to facilitate
this implementation.

It is safe to do so because a vIOMMU will be able to fail in set_iommu_device()
call, if something else related to the VFIO device or vIOMMU isn't compatible.

Suggested-by: Nicolin Chen <nicolinc@nvidia.com>
Suggested-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20260106061304.314546-9-zhenzhong.duan@intel.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>
---
 include/hw/vfio/vfio-device.h |  2 ++
 hw/vfio/device.c              | 12 ++++++++++++
 hw/vfio/iommufd.c             |  9 +++++++++
 3 files changed, 23 insertions(+)

diff --git a/include/hw/vfio/vfio-device.h b/include/hw/vfio/vfio-device.h
index 0fe6c60ba2d65b0ef5de5ef0c75c43cfa8e89352..0bc877ff62f9473d49782ce7c2ee1eb943175821 100644
--- a/include/hw/vfio/vfio-device.h
+++ b/include/hw/vfio/vfio-device.h
@@ -257,6 +257,8 @@ void vfio_device_prepare(VFIODevice *vbasedev, VFIOContainer *bcontainer,
 
 void vfio_device_unprepare(VFIODevice *vbasedev);
 
+bool vfio_device_get_viommu_flags_want_nesting(VFIODevice *vbasedev);
+
 int vfio_device_get_region_info(VFIODevice *vbasedev, int index,
                                 struct vfio_region_info **info);
 int vfio_device_get_region_info_type(VFIODevice *vbasedev, uint32_t type,
diff --git a/hw/vfio/device.c b/hw/vfio/device.c
index 19d1236ed750d482280225b1db3e4f5c13a39a89..100532f35d9ad14bf14c8d6d783f7b06aa85245a 100644
--- a/hw/vfio/device.c
+++ b/hw/vfio/device.c
@@ -23,6 +23,7 @@
 
 #include "hw/vfio/vfio-device.h"
 #include "hw/vfio/pci.h"
+#include "hw/core/iommu.h"
 #include "hw/core/hw-error.h"
 #include "trace.h"
 #include "qapi/error.h"
@@ -515,6 +516,17 @@ void vfio_device_unprepare(VFIODevice *vbasedev)
     vbasedev->bcontainer = NULL;
 }
 
+bool vfio_device_get_viommu_flags_want_nesting(VFIODevice *vbasedev)
+{
+    VFIOPCIDevice *vdev = vfio_pci_from_vfio_device(vbasedev);
+
+    if (vdev) {
+        return !!(pci_device_get_viommu_flags(PCI_DEVICE(vdev)) &
+                  VIOMMU_FLAG_WANT_NESTING_PARENT);
+    }
+    return false;
+}
+
 /*
  * Traditional ioctl() based io
  */
diff --git a/hw/vfio/iommufd.c b/hw/vfio/iommufd.c
index 32e8615ad3b41bdbeb6426c2542f445d9f31f2f5..e5328c63a366076199015b25f55d273b91a4327a 100644
--- a/hw/vfio/iommufd.c
+++ b/hw/vfio/iommufd.c
@@ -361,6 +361,15 @@ static bool iommufd_cdev_autodomains_get(VFIODevice *vbasedev,
         flags = IOMMU_HWPT_ALLOC_DIRTY_TRACKING;
     }
 
+    /*
+     * If vIOMMU requests VFIO's cooperation to create nesting parent HWPT,
+     * force to create it so that it could be reused by vIOMMU to create
+     * nested HWPT.
+     */
+    if (vfio_device_get_viommu_flags_want_nesting(vbasedev)) {
+        flags |= IOMMU_HWPT_ALLOC_NEST_PARENT;
+    }
+
     if (cpr_is_incoming()) {
         hwpt_id = vbasedev->cpr.hwpt_id;
         goto skip_alloc;
-- 
2.52.0