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Date: Tue, 25 Apr 2017 17:29:20 -0300
From: Eduardo Habkost <ehabkost@redhat.com>
Message-ID: <20170425202920.GQ3482@thinpad.lan.raisama.net>
References: <20170424130355.31324-1-marcandre.lureau@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] RFC: vmcoreinfo device
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To: =?iso-8859-1?Q?Marc-Andr=E9?= Lureau <marcandre.lureau@redhat.com>
Cc: qemu-devel@nongnu.org, pbonzini@redhat.com, lersek@redhat.com, anderson@redhat.com, "Michael S. Tsirkin" <mst@redhat.com>, Igor Mammedov <imammedo@redhat.com>, Richard Henderson <rth@twiddle.net>

On Mon, Apr 24, 2017 at 05:03:55PM +0400, Marc-Andr=E9 Lureau wrote:
[...]
> diff --git a/include/hw/compat.h b/include/hw/compat.h
> index 5d5be91daf..d0c9b71902 100644
> --- a/include/hw/compat.h
> +++ b/include/hw/compat.h
> @@ -135,6 +135,10 @@
>          .driver   =3D "vmgenid",\
>          .property =3D "x-write-pointer-available",\
>          .value    =3D "off",\
> +    },{\
> +        .driver   =3D "vmcoreinfo",\
> +        .property =3D "x-write-pointer-available",\
> +        .value    =3D "off",\
>      },

My first reaction to this was "we don't need this compat property, becaus=
e the
device didn't even exist in QEMU 2.4".

But then I read commit f2a1ae45d8ec5ad494e66a9234499a2e0fbf4b40 and now I=
 see
why this is required: this is a compat property whose sole function is to
prevent the device from being instantiated.

Instead of requiring an extra compat property, I suggest just checking if
fw_cfg has DMA enabled. e.g.:

 static void vmgenid_realize(DeviceState *dev, Error **errp)
 {
     VmGenIdState *vms =3D VMGENID(dev);
+    FWCfgState *fw_cfg =3D FW_CFG(object_resolve_path_type("", TYPE_FW_C=
FG, NULL));

-    if (!vms->write_pointer_available) {
+    if (!fw_cfg || !fw_cfg_dma_enabled(fw_cfg)) {
         error_setg(errp, "%s requires DMA write support in fw_cfg, "
                    "which this machine type does not provide", VMGENID_D=
EVICE);
         return;


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=3Doff
  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=3Don
  [boots normally]
  $

--=20
Eduardo