diff --git a/OvmfPkg/Library/PciHostBridgeLib/PciHostBridgeLib.c b/OvmfPkg/Library/PciHostBridgeLib/PciHostBridgeLib.c
index 3752993f65..2f22d637ae 100644
--- a/OvmfPkg/Library/PciHostBridgeLib/PciHostBridgeLib.c
+++ b/OvmfPkg/Library/PciHostBridgeLib/PciHostBridgeLib.c
@@ -14,14 +14,75 @@
 
 **/
 #include <PiDxe.h>
-#include <Library/PciHostBridgeLib.h>
+
+#include <IndustryStandard/Pci.h>
+
 #include <Library/DebugLib.h>
+#include <Library/MemoryAllocationLib.h>
+#include <Library/PciHostBridgeLib.h>
+#include <Library/PciLib.h>
+#include <Library/QemuFwCfgLib.h>
+
 
 GLOBAL_REMOVE_IF_UNREFERENCED
 CHAR16 *mPciHostBridgeLibAcpiAddressSpaceTypeStr[] = {
   L"Mem", L"I/O", L"Bus"
 };
 
+
+/**
+  Initialize a PCI_ROOT_BRIDGE structure.
+
+  param[in]  RootBusNumber     The bus number to store in RootBus.
+
+  param[in]  MaxSubBusNumber   The inclusive maximum bus number that can be
+                               assigned to any subordinate bus found behind any
+                               PCI bridge hanging off this root bus.
+
+                               The caller is repsonsible for ensuring that
+                               RootBusNumber <= MaxSubBusNumber. If
+                               RootBusNumber equals MaxSubBusNumber, then the
+                               root bus has no room for subordinate buses.
+
+  param[out] RootBus           The PCI_ROOT_BRIDGE structure (allocated by the
+                               caller) that should be filled in by this
+                               function.
+
+  @retval EFI_SUCCESS           Initialization successful. A device path
+                                consisting of an ACPI device path node, with
+                                UID = RootBusNumber, has been allocated and
+                                linked into RootBus.
+
+  @retval EFI_OUT_OF_RESOURCES  Memory allocation failed.
+**/
+STATIC
+EFI_STATUS
+InitRootBridge (
+  IN  UINT8           RootBusNumber,
+  IN  UINT8           MaxSubBusNumber,
+  OUT PCI_ROOT_BRIDGE *RootBus
+  )
+{
+  return EFI_OUT_OF_RESOURCES;
+}
+
+
+/**
+  Uninitialize a PCI_ROOT_BRIDGE structure set up with InitRootBridge().
+
+  param[in] RootBus  The PCI_ROOT_BRIDGE structure, allocated by the caller and
+                     initialized with InitRootBridge(), that should be
+                     uninitialized. This function doesn't free RootBus.
+**/
+STATIC
+VOID
+UninitRootBridge (
+  IN PCI_ROOT_BRIDGE *RootBus
+  )
+{
+}
+
+
 /**
   Return all the root bridge instances in an array.
 
@@ -37,10 +98,109 @@ PciHostBridgeGetRootBridges (
   UINTN *Count
   )
 {
+  EFI_STATUS           Status;
+  FIRMWARE_CONFIG_ITEM FwCfgItem;
+  UINTN                FwCfgSize;
+  UINT64               ExtraRootBridges;
+  PCI_ROOT_BRIDGE      *Bridges;
+  UINTN                Initialized;
+  UINTN                LastRootBridgeNumber;
+  UINTN                RootBridgeNumber;
+
   *Count = 0;
+
+  //
+  // QEMU provides the number of extra root buses, shortening the exhaustive
+  // search below. If there is no hint, the feature is missing.
+  //
+  Status = QemuFwCfgFindFile ("etc/extra-pci-roots", &FwCfgItem, &FwCfgSize);
+  if (EFI_ERROR (Status) || FwCfgSize != sizeof ExtraRootBridges) {
+    ExtraRootBridges = 0;
+  } else {
+    QemuFwCfgSelectItem (FwCfgItem);
+    QemuFwCfgReadBytes (FwCfgSize, &ExtraRootBridges);
+
+    if (ExtraRootBridges > PCI_MAX_BUS) {
+      DEBUG ((EFI_D_ERROR, "%a: invalid count of extra root buses (%Lu) "
+        "reported by QEMU\n", __FUNCTION__, ExtraRootBridges));
+      return NULL;
+    }
+    DEBUG ((EFI_D_INFO, "%a: %Lu extra root buses reported by QEMU\n",
+      __FUNCTION__, ExtraRootBridges));
+  }
+
+  //
+  // Allocate the "main" root bridge, and any extra root bridges.
+  //
+  Bridges = AllocatePool ((1 + (UINTN)ExtraRootBridges) * sizeof *Bridges);
+  if (Bridges == NULL) {
+    DEBUG ((EFI_D_ERROR, "%a: %r\n", __FUNCTION__, EFI_OUT_OF_RESOURCES));
+    return NULL;
+  }
+  Initialized = 0;
+
+  //
+  // The "main" root bus is always there.
+  //
+  LastRootBridgeNumber = 0;
+
+  //
+  // Scan all other root buses. If function 0 of any device on a bus returns a
+  // VendorId register value different from all-bits-one, then that bus is
+  // alive.
+  //
+  for (RootBridgeNumber = 1;
+       RootBridgeNumber <= PCI_MAX_BUS && Initialized < ExtraRootBridges;
+       ++RootBridgeNumber) {
+    UINTN Device;
+
+    for (Device = 0; Device <= PCI_MAX_DEVICE; ++Device) {
+      if (PciRead16 (PCI_LIB_ADDRESS (RootBridgeNumber, Device, 0,
+                       PCI_VENDOR_ID_OFFSET)) != MAX_UINT16) {
+        break;
+      }
+    }
+    if (Device <= PCI_MAX_DEVICE) {
+      //
+      // Found the next root bus. We can now install the *previous* one,
+      // because now we know how big a bus number range *that* one has, for any
+      // subordinate buses that might exist behind PCI bridges hanging off it.
+      //
+      Status = InitRootBridge ((UINT8)LastRootBridgeNumber,
+                 (UINT8)(RootBridgeNumber - 1), &Bridges[Initialized]);
+      if (EFI_ERROR (Status)) {
+        goto FreeBridges;
+      }
+      ++Initialized;
+      LastRootBridgeNumber = RootBridgeNumber;
+    }
+  }
+
+  //
+  // Install the last root bus (which might be the only, ie. main, root bus, if
+  // we've found no extra root buses).
+  //
+  Status = InitRootBridge ((UINT8)LastRootBridgeNumber, PCI_MAX_BUS,
+             &Bridges[Initialized]);
+  if (EFI_ERROR (Status)) {
+    goto FreeBridges;
+  }
+  ++Initialized;
+
+  *Count = Initialized;
+  return Bridges;
+
+FreeBridges:
+  while (Initialized > 0) {
+    --Initialized;
+    UninitRootBridge (&Bridges[Initialized]);
+  }
+
+  FreePool (Bridges);
   return NULL;
 }
 
+
 /**
   Free the root bridge instances array returned from
   PciHostBridgeGetRootBridges().
@@ -58,6 +218,7 @@ PciHostBridgeFreeRootBridges (
   return;
 }
 
+
 /**
   Inform the platform that the resource conflict happens.
 
diff --git a/OvmfPkg/Library/PciHostBridgeLib/PciHostBridgeLib.inf b/OvmfPkg/Library/PciHostBridgeLib/PciHostBridgeLib.inf
index 1f3930195a..b836931912 100644
--- a/OvmfPkg/Library/PciHostBridgeLib/PciHostBridgeLib.inf
+++ b/OvmfPkg/Library/PciHostBridgeLib/PciHostBridgeLib.inf
@@ -34,5 +34,12 @@
   PciHostBridgeLib.c
 
 [Packages]
-  MdePkg/MdePkg.dec
   MdeModulePkg/MdeModulePkg.dec
+  MdePkg/MdePkg.dec
+  OvmfPkg/OvmfPkg.dec
+
+[LibraryClasses]
+  DebugLib
+  MemoryAllocationLib
+  PciLib
+  QemuFwCfgLib