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>
---
 include/hw/acpi/bios-linker-loader.h |  2 ++
 include/hw/compat.h                  |  4 ----
 hw/acpi/bios-linker-loader.c         | 10 ++++++++++
 hw/acpi/vmgenid.c                    |  9 +--------
 4 files changed, 13 insertions(+), 12 deletions(-)

diff --git a/include/hw/acpi/bios-linker-loader.h b/include/hw/acpi/bios-linker-loader.h
index efe17b0b9c..a711dbced8 100644
--- a/include/hw/acpi/bios-linker-loader.h
+++ b/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,
diff --git a/include/hw/compat.h b/include/hw/compat.h
index 08f36004da..f414786604 100644
--- a/include/hw/compat.h
+++ b/include/hw/compat.h
@@ -150,10 +150,6 @@
         .driver   = "fw_cfg_io",\
         .property = "dma_enabled",\
         .value    = "off",\
-    },{\
-        .driver   = "vmgenid",\
-        .property = "x-write-pointer-available",\
-        .value    = "off",\
     },
 
 #define HW_COMPAT_2_3 \
diff --git a/hw/acpi/bios-linker-loader.c b/hw/acpi/bios-linker-loader.c
index 046183a0f1..388d932538 100644
--- a/hw/acpi/bios-linker-loader.c
+++ b/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.
  *
diff --git a/hw/acpi/vmgenid.c b/hw/acpi/vmgenid.c
index a32b847fe0..ab5da293fd 100644
--- a/hw/acpi/vmgenid.c
+++ b/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;
 
     object_class_property_add_str(klass, VMGENID_GUID, NULL,
                                   vmgenid_set_guid, NULL);
-- 
2.13.1.395.gf7b71de06

On 07/06/17 12:16, Marc-André Lureau wrote:
> 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>
> ---
>  include/hw/acpi/bios-linker-loader.h |  2 ++
>  include/hw/compat.h                  |  4 ----
>  hw/acpi/bios-linker-loader.c         | 10 ++++++++++
>  hw/acpi/vmgenid.c                    |  9 +--------
>  4 files changed, 13 insertions(+), 12 deletions(-)
> 
> diff --git a/include/hw/acpi/bios-linker-loader.h b/include/hw/acpi/bios-linker-loader.h
> index efe17b0b9c..a711dbced8 100644
> --- a/include/hw/acpi/bios-linker-loader.h
> +++ b/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,
> diff --git a/include/hw/compat.h b/include/hw/compat.h
> index 08f36004da..f414786604 100644
> --- a/include/hw/compat.h
> +++ b/include/hw/compat.h
> @@ -150,10 +150,6 @@
>          .driver   = "fw_cfg_io",\
>          .property = "dma_enabled",\
>          .value    = "off",\
> -    },{\
> -        .driver   = "vmgenid",\
> -        .property = "x-write-pointer-available",\
> -        .value    = "off",\
>      },
>  
>  #define HW_COMPAT_2_3 \
> diff --git a/hw/acpi/bios-linker-loader.c b/hw/acpi/bios-linker-loader.c
> index 046183a0f1..388d932538 100644
> --- a/hw/acpi/bios-linker-loader.c
> +++ b/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()
> +*/

The closing "*/" is not correctly indented.

With that fixed,

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Thanks
Laszlo

> +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.
>   *
> diff --git a/hw/acpi/vmgenid.c b/hw/acpi/vmgenid.c
> index a32b847fe0..ab5da293fd 100644
> --- a/hw/acpi/vmgenid.c
> +++ b/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;
>  
>      object_class_property_add_str(klass, VMGENID_GUID, NULL,
>                                    vmgenid_set_guid, NULL);
>