Fix pc migration from qemu <= 1.5

The following commit introduced a migration incompatibility:

commit 568f0690fd
Author: David Gibson <david@gibson.dropbear.id.au>
Date:   Thu Jun 6 18:48:49 2013 +1000

    pci: Replace pci_find_domain() with more general pci_root_bus_path()

The issue is that i440fx savevm idstr went from 0000:00:00.0/I440FX to
0000:00.0/I440FX. Unfortunately we are stuck with the breakage for
1.6 machine types.

Add a compat property to maintain the busted idstr for the 1.6 machine
types, but revert to the old style format for 1.7+, and <= 1.5.

Tested with migration from qemu 1.5, qemu 1.6, and qemu.git.

Cc: qemu-stable@nongnu.org
Signed-off-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/pci-host/piix.c

@@ -48,6 +48,7 @@ typedef struct I440FXState {
     PCIHostState parent_obj;
     PcPciInfo pci_info;
     uint64_t pci_hole64_size;
+    uint32_t short_root_bus;
 } I440FXState;
 
 #define PIIX_NUM_PIC_IRQS       16      /* i8259 * 2 */
@@ -720,13 +721,19 @@ static const TypeInfo i440fx_info = {
 static const char *i440fx_pcihost_root_bus_path(PCIHostState *host_bridge,
                                                 PCIBus *rootbus)
 {
+    I440FXState *s = I440FX_PCI_HOST_BRIDGE(host_bridge);
+
     /* For backwards compat with old device paths */
-    return "0000";
+    if (s->short_root_bus) {
+        return "0000";
+    }
+    return "0000:00";
 }
 
 static Property i440fx_props[] = {
     DEFINE_PROP_SIZE(PCI_HOST_PROP_PCI_HOLE64_SIZE, I440FXState,
                      pci_hole64_size, DEFAULT_PCI_HOLE64_SIZE),
+    DEFINE_PROP_UINT32("short_root_bus", I440FXState, short_root_bus, 0),
     DEFINE_PROP_END_OF_LIST(),
 };
 

hw/pci-host/q35.c

@@ -61,8 +61,13 @@ static void q35_host_realize(DeviceState *dev, Error **errp)
 static const char *q35_host_root_bus_path(PCIHostState *host_bridge,
                                           PCIBus *rootbus)
 {
-    /* For backwards compat with old device paths */
-    return "0000";
+    Q35PCIHost *s = Q35_HOST_DEVICE(host_bridge);
+
+     /* For backwards compat with old device paths */
+    if (s->mch.short_root_bus) {
+        return "0000";
+    }
+    return "0000:00";
 }
 
 static void q35_host_get_pci_hole_start(Object *obj, Visitor *v,
@@ -124,6 +129,7 @@ static Property mch_props[] = {
                         MCH_HOST_BRIDGE_PCIEXBAR_DEFAULT),
     DEFINE_PROP_SIZE(PCI_HOST_PROP_PCI_HOLE64_SIZE, Q35PCIHost,
                      mch.pci_hole64_size, DEFAULT_PCI_HOLE64_SIZE),
+    DEFINE_PROP_UINT32("short_root_bus", Q35PCIHost, mch.short_root_bus, 0),
     DEFINE_PROP_END_OF_LIST(),
 };
 

include/hw/i386/pc.h

@@ -260,6 +260,14 @@ int e820_add_entry(uint64_t, uint64_t, uint32_t);
             .driver   = "qemu32-" TYPE_X86_CPU,\
             .property = "model",\
             .value    = stringify(3),\
+        },{\
+            .driver   = "i440FX-pcihost",\
+            .property = "short_root_bus",\
+            .value    = stringify(1),\
+        },{\
+            .driver   = "q35-pcihost",\
+            .property = "short_root_bus",\
+            .value    = stringify(1),\
         }
 
 #define PC_COMPAT_1_5 \
@@ -296,6 +304,14 @@ int e820_add_entry(uint64_t, uint64_t, uint32_t);
             .driver = TYPE_X86_CPU,\
             .property = "pmu",\
             .value = "on",\
+        },{\
+            .driver   = "i440FX-pcihost",\
+            .property = "short_root_bus",\
+            .value    = stringify(0),\
+        },{\
+            .driver   = "q35-pcihost",\
+            .property = "short_root_bus",\
+            .value    = stringify(0),\
         }
 
 #define PC_COMPAT_1_4 \

include/hw/pci-host/q35.h

@@ -61,6 +61,7 @@ typedef struct MCHPCIState {
     ram_addr_t above_4g_mem_size;
     uint64_t pci_hole64_size;
     PcGuestInfo *guest_info;
+    uint32_t short_root_bus;
 } MCHPCIState;
 
 typedef struct Q35PCIHost {