Re: [Qemu-devel] [PATCH 1/2] hw/acpi/vmgenid: prevent device realization on pre-2.9 machine types

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Mar 20, 2017 at 12:59:50PM +0100, Laszlo Ersek wrote:
> The WRITE_POINTER linker/loader command that underlies VMGENID depends on
> commit baf2d5bfbac0 ("fw-cfg: support writeable blobs", 2017-01-12). That
> commit is not available in 2.8.
> 
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Ben Warren <ben@skyportsystems.com>
> Cc: Igor Mammedov <imammedo@redhat.com>
> Signed-off-by: Laszlo Ersek <lersek@redhat.com>

I don't understand why do we want to add code to break this
intentionally. What issue does this fix?

> ---
>  include/hw/acpi/vmgenid.h |  1 +
>  include/hw/compat.h       |  4 ++++
>  hw/acpi/vmgenid.c         | 14 ++++++++++++++
>  3 files changed, 19 insertions(+)
> 
> diff --git a/include/hw/acpi/vmgenid.h b/include/hw/acpi/vmgenid.h
> index db7fa0e63303..8578476baebe 100644
> --- a/include/hw/acpi/vmgenid.h
> +++ b/include/hw/acpi/vmgenid.h
> @@ -21,6 +21,7 @@ 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;
>  
>  static inline Object *find_vmgenid_dev(void)
> diff --git a/include/hw/compat.h b/include/hw/compat.h
> index fc8c3e060007..c9caa8cd1bdd 100644
> --- a/include/hw/compat.h
> +++ b/include/hw/compat.h
> @@ -42,6 +42,10 @@
>          .driver   = "isa-cirrus-vga",\
>          .property = "vgamem_mb",\
>          .value    = "8",\
> +    },{\
> +        .driver   = "vmgenid",\
> +        .property = "x-write-pointer-available",\
> +        .value    = "off",\
>      },
>  
>  #define HW_COMPAT_2_7 \
> diff --git a/hw/acpi/vmgenid.c b/hw/acpi/vmgenid.c
> index 7a3ad17d66ef..c3ddcc8e7cb0 100644
> --- a/hw/acpi/vmgenid.c
> +++ b/hw/acpi/vmgenid.c
> @@ -205,9 +205,22 @@ 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) {
> +        error_setg(errp, "%s requires DMA write support in fw_cfg, "
> +                   "which this machine type does not provide", VMGENID_DEVICE);
> +        return;
> +    }
> +
>      qemu_register_reset(vmgenid_handle_reset, vms);
>  }
>  
> @@ -218,6 +231,7 @@ 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);
> -- 
> 2.9.3
>