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Re: [Xen-devel] [PATCH v9 12/14] tools/libxl: read from config if passing through primary GPU

Thursday, July 3, 2014, 12:30:13 PM, you wrote:

>>>> On 02.07.14 at 21:00, <linux@xxxxxxxxxxxxxx> wrote:
>> Wednesday, July 2, 2014, 8:42:21 PM, you wrote:
>>> --- a/docs/man/xl.cfg.pod.5
>>> +++ b/docs/man/xl.cfg.pod.5
>>> @@ -601,6 +601,13 @@ More information about Xen gfx_passthru feature is 
>>> available
>>>  on the XenVGAPassthrough L<http://wiki.xen.org/wiki/XenVGAPassthrough>
>>>  wiki page.
>>> +=item B<gfx_passthru_primary=BOOLEAN>
>>> +
>>> +If set to true, indicates that one of the GPUs passed through to the
>>> +domain is the primary one; therefore allows to the domain's stubdoman
>>> +access permissions for the VGA framebuffer. Note that this option
>>> +defaults to true.
>>> +
>> Wouldn't it make sense to make it a pci <BDF> instead of boolean, 
>> so that instead of: "one of the GPU's passed is the primary",
>> you could actually specify: "this *specific* GPU passed through i would like 
>> to be the primary" ?

> No. You can't just make some card primary (with "primary" really
> meaning accessible via the legacy VGA interface). Within a host
> there's exactly one card that can be primary. If that card gets
> passed through, it can also be primary in the guest. Any other
> card, not having the legacy VGA ports routed through the
> corresponding circuitry, can't all of the sudden become the
> primary one in a guest.

Is that also true for a HVM guest ?
- aren't the legacy VGA ports emulated without passthrough (by hvmloader / 
  seabios, with the cirrus or stdvga)
- couldn't they be rerouted to a device that is passed through ?

Or would that make the VGA card do legacy stuff on the host instead of in the 
guest (since it's lowlevel legacy stuff) ?

> Consequently this shouldn't be a config file setting, but something
> that gets worked out by looking at the PCI config spaces of the
> passed through card and eventual upstream bridges.

> Jan

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