vmgenid: replace x-write-pointer-available hack

This compat property sole function is to prevent the device from being
instantiated. Instead of requiring an extra compat property, check if
fw_cfg has DMA enabled.

fw_cfg is a built-in device that is initialized very early by the
machine init code.  We have at least one other device that also
assumes fw_cfg_find() can be safely used on realize: pvpanic.

This has the additional benefit of handling other cases properly, like:

  $ qemu-system-x86_64 -device vmgenid -machine none
  qemu-system-x86_64: -device vmgenid: vmgenid requires DMA write support in fw_cfg, which this machine type does not provide
  $ qemu-system-x86_64 -device vmgenid -machine pc-i440fx-2.9 -global fw_cfg.dma_enabled=off
  qemu-system-x86_64: -device vmgenid: vmgenid requires DMA write support in fw_cfg, which this machine type does not provide
  $ qemu-system-x86_64 -device vmgenid -machine pc-i440fx-2.6 -global fw_cfg.dma_enabled=on
  [boots normally]

Suggested-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Ben Warren <ben@skyportsystems.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/acpi/bios-linker-loader.c

@@ -168,6 +168,16 @@ bios_linker_find_file(const BIOSLinker *linker, const char *name)
     return NULL;
 }
 
+/*
+ * board code must realize fw_cfg first, as a fixed device, before
+ * another device realize function call bios_linker_loader_can_write_pointer()
+ */
+bool bios_linker_loader_can_write_pointer(void)
+{
+    FWCfgState *fw_cfg = fw_cfg_find();
+    return fw_cfg && fw_cfg_dma_enabled(fw_cfg);
+}
+
 /*
  * bios_linker_loader_alloc: ask guest to load file into guest memory.
  *

hw/acpi/vmgenid.c

@@ -205,17 +205,11 @@ static void vmgenid_handle_reset(void *opaque)
     memset(vms->vmgenid_addr_le, 0, ARRAY_SIZE(vms->vmgenid_addr_le));
 }
 
-static Property vmgenid_properties[] = {
-    DEFINE_PROP_BOOL("x-write-pointer-available", VmGenIdState,
-                     write_pointer_available, true),
-    DEFINE_PROP_END_OF_LIST(),
-};
-
 static void vmgenid_realize(DeviceState *dev, Error **errp)
 {
     VmGenIdState *vms = VMGENID(dev);
 
-    if (!vms->write_pointer_available) {
+    if (!bios_linker_loader_can_write_pointer()) {
         error_setg(errp, "%s requires DMA write support in fw_cfg, "
                    "which this machine type does not provide", VMGENID_DEVICE);
         return;
@@ -239,7 +233,6 @@ static void vmgenid_device_class_init(ObjectClass *klass, void *data)
     dc->vmsd = &vmstate_vmgenid;
     dc->realize = vmgenid_realize;
     dc->hotpluggable = false;
-    dc->props = vmgenid_properties;
     set_bit(DEVICE_CATEGORY_MISC, dc->categories);
 
     object_class_property_add_str(klass, VMGENID_GUID, NULL,

include/hw/acpi/bios-linker-loader.h

@@ -7,6 +7,8 @@ typedef struct BIOSLinker {
     GArray *file_list;
 } BIOSLinker;
 
+bool bios_linker_loader_can_write_pointer(void);
+
 BIOSLinker *bios_linker_loader_init(void);
 
 void bios_linker_loader_alloc(BIOSLinker *linker,

include/hw/acpi/vmgenid.h

@@ -21,7 +21,6 @@ typedef struct VmGenIdState {
     DeviceClass parent_obj;
     QemuUUID guid;                /* The 128-bit GUID seen by the guest */
     uint8_t vmgenid_addr_le[8];   /* Address of the GUID (little-endian) */
-    bool write_pointer_available;
 } VmGenIdState;
 
 /* returns NULL unless there is exactly one device */

include/hw/compat.h

@@ -153,10 +153,6 @@
         .driver   = "fw_cfg_io",\
         .property = "dma_enabled",\
         .value    = "off",\
-    },{\
-        .driver   = "vmgenid",\
-        .property = "x-write-pointer-available",\
-        .value    = "off",\
     },
 
 #define HW_COMPAT_2_3 \