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From: "Chen, Tiejun" <tiejun.chen@intel.com>
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Subject: Re: [Qemu-devel] [v6][PATCH 05/10] xen,
 gfx passthrough: basic graphics passthrough support
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To: Gerd Hoffmann <kraxel@redhat.com>
Cc: mst@redhat.com, allen.m.kay@intel.com, qemu-devel@nongnu.org, aliguori@amazon.com, yang.z.zhang@intel.com, pbonzini@redhat.com, rth@twiddle.net

On 2015/1/20 16:14, Gerd Hoffmann wrote:
> On Di, 2015-01-20 at 11:14 +0800, Chen, Tiejun wrote:
>> On 2015/1/19 19:45, Gerd Hoffmann wrote:
>>> On Mo, 2015-01-19 at 17:28 +0800, Tiejun Chen wrote:
>>>> +DEF("gfx_passthru", 0, QEMU_OPTION_gfx_passthru,
>>>> +    "-gfx_passthru   enable Intel IGD passthrough by XEN\n",
>>>> +    QEMU_ARCH_ALL)
>>>> +STEXI
>>>> +@item -gfx_passthru
>>>> +@findex -gfx_passthru
>>>> +Enable Intel IGD passthrough by XEN
>>>> +ETEXI
>>>
>>> Make that a machine option, i.e. "-machine pc,igd-passthru=on"?
>>
>> Yeah but I think we need "-machine xenfv,igd-passthru=on" here.
>
> IIRC xen decided to stop using xenfv and use pc-$version instead (with
> $version being what was current at release time, 1.6 for xen 4.4 I
> think, to avoid surprises like the address space layout changes in more
> recent machine types).
>

To be more exact, 'xen_platform_pci' should determine this at this point,

         if (!libxl_defbool_val(b_info->u.hvm.xen_platform_pci)) {
             /* Switching here to the machine "pc" which does not add
              * the xen-platform device instead of the default "xenfv" 
machine.
              */
             machinearg = libxl__sprintf(gc, "pc,accel=xen");
         } else {
             machinearg = libxl__sprintf(gc, "xenfv");
         }

But you may remember, in our case we always set 'xen_platform_pci=0' 
since we need to release slot 2 for IGD. So finally we really go pc case.

Anyway this means something should be changed to pass such a new machine 
property in Xen side. And I'll send a patch to address this firstly, 
then go back here.

Thanks
Tiejun