When guest enables scalable mode and setup first stage page table, we don't
want to use IOMMU MR but rather continue using the system MR for IOMMUFD
backed host device.

Then default HWPT in VFIO contains GPA->HPA mappings which could be reused
as nesting parent HWPT to construct nested HWPT in vIOMMU.

Suggested-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
---
 hw/i386/intel_iommu.c | 36 ++++++++++++++++++++++++++++++++++--
 1 file changed, 34 insertions(+), 2 deletions(-)

diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
index 4c83578c54..ce4c54165e 100644
--- a/hw/i386/intel_iommu.c
+++ b/hw/i386/intel_iommu.c
@@ -41,6 +41,7 @@
 #include "migration/misc.h"
 #include "migration/vmstate.h"
 #include "trace.h"
+#include "system/iommufd.h"
 
 /* context entry operations */
 #define PASID_0    0
@@ -1713,6 +1714,24 @@ static bool vtd_dev_pt_enabled(IntelIOMMUState *s, VTDContextEntry *ce,
 
 }
 
+static VTDHostIOMMUDevice *vtd_find_hiod_iommufd(VTDAddressSpace *as)
+{
+    IntelIOMMUState *s = as->iommu_state;
+    struct vtd_as_key key = {
+        .bus = as->bus,
+        .devfn = as->devfn,
+    };
+    VTDHostIOMMUDevice *vtd_hiod = g_hash_table_lookup(s->vtd_host_iommu_dev,
+                                                       &key);
+
+    if (vtd_hiod && vtd_hiod->hiod &&
+        object_dynamic_cast(OBJECT(vtd_hiod->hiod),
+                            TYPE_HOST_IOMMU_DEVICE_IOMMUFD)) {
+        return vtd_hiod;
+    }
+    return NULL;
+}
+
 static bool vtd_as_pt_enabled(VTDAddressSpace *as)
 {
     IntelIOMMUState *s;
@@ -1738,12 +1757,25 @@ static bool vtd_as_pt_enabled(VTDAddressSpace *as)
 /* Return whether the device is using IOMMU translation. */
 static bool vtd_switch_address_space(VTDAddressSpace *as)
 {
+    IntelIOMMUState *s;
     bool use_iommu, pt;
 
     assert(as);
 
-    use_iommu = as->iommu_state->dmar_enabled && !vtd_as_pt_enabled(as);
-    pt = as->iommu_state->dmar_enabled && vtd_as_pt_enabled(as);
+    s = as->iommu_state;
+    use_iommu = s->dmar_enabled && !vtd_as_pt_enabled(as);
+    pt = s->dmar_enabled && vtd_as_pt_enabled(as);
+
+    /*
+     * When guest enables scalable mode and sets up first stage page table,
+     * we stick to system MR for IOMMUFD backed host device. Then its
+     * default hwpt contains GPA->HPA mappings which is used directly if
+     * PGTT=PT and used as nesting parent if PGTT=FST. Otherwise fall back
+     * to original processing.
+     */
+    if (s->root_scalable && s->fsts && vtd_find_hiod_iommufd(as)) {
+        use_iommu = false;
+    }
 
     trace_vtd_switch_address_space(pci_bus_num(as->bus),
                                    VTD_PCI_SLOT(as->devfn),
-- 
2.47.1