OvmfPkg/Microvm/fdt: load fdt from fw_cfg

Needed for hardware detection: virtio-mmio devices for now,
later also pcie root bridge.

Depends on patched qemu which actually provides an fdt:
https://gitlab.com/kraxel/qemu/-/commits/sirius/microvm-device-tree

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3689
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>

OvmfPkg/PlatformPei/Platform.c

@@ -321,6 +321,50 @@ PciExBarInitialization (
     );
 }
 
+VOID
+MicrovmInitialization (
+  VOID
+  )
+{
+  FIRMWARE_CONFIG_ITEM  FdtItem;
+  UINTN                 FdtSize;
+  UINTN                 FdtPages;
+  EFI_STATUS            Status;
+  UINT64                *FdtHobData;
+  VOID                  *NewBase;
+
+  Status = QemuFwCfgFindFile ("etc/fdt", &FdtItem, &FdtSize);
+  if (EFI_ERROR (Status)) {
+    DEBUG ((DEBUG_INFO, "%a: no etc/fdt found in fw_cfg\n", __FUNCTION__));
+    return;
+  }
+
+  FdtPages = EFI_SIZE_TO_PAGES (FdtSize);
+  NewBase  = AllocatePages (FdtPages);
+  if (NewBase == NULL) {
+    DEBUG ((DEBUG_INFO, "%a: AllocatePages failed\n", __FUNCTION__));
+    return;
+  }
+
+  QemuFwCfgSelectItem (FdtItem);
+  QemuFwCfgReadBytes (FdtSize, NewBase);
+
+  FdtHobData = BuildGuidHob (&gFdtHobGuid, sizeof (*FdtHobData));
+  if (FdtHobData == NULL) {
+    DEBUG ((DEBUG_INFO, "%a: BuildGuidHob failed\n", __FUNCTION__));
+    return;
+  }
+
+  DEBUG ((
+    DEBUG_INFO,
+    "%a: fdt at 0x%x (size %d)\n",
+    __FUNCTION__,
+    NewBase,
+    FdtSize
+    ));
+  *FdtHobData = (UINTN)NewBase;
+}
+
 VOID
 MiscInitialization (
   VOID
@@ -368,6 +412,7 @@ MiscInitialization (
       break;
     case 0xffff: /* microvm */
       DEBUG ((DEBUG_INFO, "%a: microvm\n", __FUNCTION__));
+      MicrovmInitialization ();
       PcdStatus = PcdSet16S (
                     PcdOvmfHostBridgePciDevId,
                     MICROVM_PSEUDO_DEVICE_ID

OvmfPkg/PlatformPei/PlatformPei.inf

@@ -44,6 +44,7 @@
 
 [Guids]
   gEfiMemoryTypeInformationGuid
+  gFdtHobGuid
 
 [LibraryClasses]
   BaseLib