intel_iommu_accel: Fail passthrough device under PCI bridge if x-flts=on

Currently we don't support nested translation for passthrough device with
emulated device under same PCI bridge, because they require different address
space when x-flts=on.

In theory, we do support if devices under same PCI bridge are all passthrough
devices. But emulated device can be hotplugged under same bridge. To simplify,
just forbid passthrough device under PCI bridge no matter if there is, or will
be emulated devices under same bridge. This is acceptable because PCIE bridge
is more popular than PCI bridge now.

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>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20260106061304.314546-11-zhenzhong.duan@intel.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

hw/i386/intel_iommu.c

@@ -4570,9 +4570,10 @@ VTDAddressSpace *vtd_find_add_as(IntelIOMMUState *s, PCIBus *bus,
     return vtd_dev_as;
 }
 
-static bool vtd_check_hiod(IntelIOMMUState *s, HostIOMMUDevice *hiod,
+static bool vtd_check_hiod(IntelIOMMUState *s, VTDHostIOMMUDevice *vtd_hiod,
                            Error **errp)
 {
+    HostIOMMUDevice *hiod = vtd_hiod->hiod;
     HostIOMMUDeviceClass *hiodc = HOST_IOMMU_DEVICE_GET_CLASS(hiod);
     int ret;
 
@@ -4596,7 +4597,7 @@ static bool vtd_check_hiod(IntelIOMMUState *s, HostIOMMUDevice *hiod,
         return true;
     }
 
-    return vtd_check_hiod_accel(s, hiod, errp);
+    return vtd_check_hiod_accel(s, vtd_hiod, errp);
 }
 
 static bool vtd_dev_set_iommu_device(PCIBus *bus, void *opaque, int devfn,
@@ -4632,7 +4633,7 @@ static bool vtd_dev_set_iommu_device(PCIBus *bus, void *opaque, int devfn,
     vtd_hiod->iommu_state = s;
     vtd_hiod->hiod = hiod;
 
-    if (!vtd_check_hiod(s, hiod, errp)) {
+    if (!vtd_check_hiod(s, vtd_hiod, errp)) {
         g_free(vtd_hiod);
         vtd_iommu_unlock(s);
         return false;

hw/i386/intel_iommu_accel.c

@@ -12,12 +12,16 @@
 #include "system/iommufd.h"
 #include "intel_iommu_internal.h"
 #include "intel_iommu_accel.h"
+#include "hw/pci/pci_bus.h"
 
-bool vtd_check_hiod_accel(IntelIOMMUState *s, HostIOMMUDevice *hiod,
+bool vtd_check_hiod_accel(IntelIOMMUState *s, VTDHostIOMMUDevice *vtd_hiod,
                           Error **errp)
 {
+    HostIOMMUDevice *hiod = vtd_hiod->hiod;
     struct HostIOMMUDeviceCaps *caps = &hiod->caps;
     struct iommu_hw_info_vtd *vtd = &caps->vendor_caps.vtd;
+    PCIBus *bus = vtd_hiod->bus;
+    PCIDevice *pdev = bus->devices[vtd_hiod->devfn];
 
     if (!object_dynamic_cast(OBJECT(hiod), TYPE_HOST_IOMMU_DEVICE_IOMMUFD)) {
         error_setg(errp, "Need IOMMUFD backend when x-flts=on");
@@ -36,6 +40,12 @@ bool vtd_check_hiod_accel(IntelIOMMUState *s, HostIOMMUDevice *hiod,
         return false;
     }
 
+    if (pci_device_get_iommu_bus_devfn(pdev, &bus, NULL, NULL)) {
+        error_setg(errp, "Host device downstream to a PCI bridge is "
+                   "unsupported when x-flts=on");
+        return false;
+    }
+
     error_setg(errp,
                "host IOMMU is incompatible with guest first stage translation");
     return false;

hw/i386/intel_iommu_accel.h

@@ -13,11 +13,11 @@
 #include CONFIG_DEVICES
 
 #ifdef CONFIG_VTD_ACCEL
-bool vtd_check_hiod_accel(IntelIOMMUState *s, HostIOMMUDevice *hiod,
+bool vtd_check_hiod_accel(IntelIOMMUState *s, VTDHostIOMMUDevice *vtd_hiod,
                           Error **errp);
 #else
 static inline bool vtd_check_hiod_accel(IntelIOMMUState *s,
-                                        HostIOMMUDevice *hiod,
+                                        VTDHostIOMMUDevice *vtd_hiod,
                                         Error **errp)
 {
     error_setg(errp, "host IOMMU cannot be checked!");