fw_cfg: unbreak migration compatibility for 2.4 and earlier machines

When I reviewed Marc's fw_cfg DMA patches, I completely missed that the
way we set dma_enabled would break migration.

Gerd explained the right way (see reference below): dma_enabled should be
set to true by default, and only true->false transitions should be
possible:

- when the user requests that with

    -global fw_cfg_mem.dma_enabled=off

  or

   -global fw_cfg_io.dma_enabled=off

  as appropriate for the platform,

- when HW_COMPAT_2_4 dictates it,

- when board code initializes fw_cfg without requesting DMA support.

Cc: Marc Marí <markmb@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Alexandre DERUMIER <aderumier@odiso.com>
Cc: qemu-stable@nongnu.org
Ref: http://thread.gmane.org/gmane.comp.emulators.qemu/390272/focus=391042
Ref: https://bugs.launchpad.net/qemu/+bug/1536487
Suggested-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-id: 1455823860-22268-1-git-send-email-lersek@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/nvram/fw_cfg.c

@@ -778,17 +778,19 @@ FWCfgState *fw_cfg_init_io_dma(uint32_t iobase, uint32_t dma_iobase,
     DeviceState *dev;
     FWCfgState *s;
     uint32_t version = FW_CFG_VERSION;
-    bool dma_enabled = dma_iobase && dma_as;
+    bool dma_requested = dma_iobase && dma_as;
 
     dev = qdev_create(NULL, TYPE_FW_CFG_IO);
     qdev_prop_set_uint32(dev, "iobase", iobase);
     qdev_prop_set_uint32(dev, "dma_iobase", dma_iobase);
-    qdev_prop_set_bit(dev, "dma_enabled", dma_enabled);
+    if (!dma_requested) {
+        qdev_prop_set_bit(dev, "dma_enabled", false);
+    }
 
     fw_cfg_init1(dev);
     s = FW_CFG(dev);
 
-    if (dma_enabled) {
+    if (s->dma_enabled) {
         /* 64 bits for the address field */
         s->dma_as = dma_as;
         s->dma_addr = 0;
@@ -814,11 +816,13 @@ FWCfgState *fw_cfg_init_mem_wide(hwaddr ctl_addr,
     SysBusDevice *sbd;
     FWCfgState *s;
     uint32_t version = FW_CFG_VERSION;
-    bool dma_enabled = dma_addr && dma_as;
+    bool dma_requested = dma_addr && dma_as;
 
     dev = qdev_create(NULL, TYPE_FW_CFG_MEM);
     qdev_prop_set_uint32(dev, "data_width", data_width);
-    qdev_prop_set_bit(dev, "dma_enabled", dma_enabled);
+    if (!dma_requested) {
+        qdev_prop_set_bit(dev, "dma_enabled", false);
+    }
 
     fw_cfg_init1(dev);
 
@@ -828,7 +832,7 @@ FWCfgState *fw_cfg_init_mem_wide(hwaddr ctl_addr,
 
     s = FW_CFG(dev);
 
-    if (dma_enabled) {
+    if (s->dma_enabled) {
         s->dma_as = dma_as;
         s->dma_addr = 0;
         sysbus_mmio_map(sbd, 2, dma_addr);
@@ -873,7 +877,7 @@ static Property fw_cfg_io_properties[] = {
     DEFINE_PROP_UINT32("iobase", FWCfgIoState, iobase, -1),
     DEFINE_PROP_UINT32("dma_iobase", FWCfgIoState, dma_iobase, -1),
     DEFINE_PROP_BOOL("dma_enabled", FWCfgIoState, parent_obj.dma_enabled,
-                     false),
+                     true),
     DEFINE_PROP_END_OF_LIST(),
 };
 
@@ -913,7 +917,7 @@ static const TypeInfo fw_cfg_io_info = {
 static Property fw_cfg_mem_properties[] = {
     DEFINE_PROP_UINT32("data_width", FWCfgMemState, data_width, -1),
     DEFINE_PROP_BOOL("dma_enabled", FWCfgMemState, parent_obj.dma_enabled,
-                     false),
+                     true),
     DEFINE_PROP_END_OF_LIST(),
 };
 

include/hw/compat.h

@@ -42,6 +42,14 @@
         .driver   = "virtio-pci",\
         .property = "migrate-extra",\
         .value    = "off",\
+    },{\
+        .driver   = "fw_cfg_mem",\
+        .property = "dma_enabled",\
+        .value    = "off",\
+    },{\
+        .driver   = "fw_cfg_io",\
+        .property = "dma_enabled",\
+        .value    = "off",\
     },
 
 #define HW_COMPAT_2_3 \