Re: [PATCH v5 10/11] arm/libxl: Emulated PCI device tree node in libxl

[Ian Jackson <iwj@xxxxxxxxxxxxxx>]

Rahul Singh writes ("Re: [PATCH v5 10/11] arm/libxl: Emulated PCI device tree 
node in libxl"):
> As Stefano suggested in another email that we can remove the vpci
> option, if we reach to conclusion that we need vpci option I will
> move it to internal structure.
...
> Yes I agree with you VPCI is necessary for hot plugged PCI device
> and once we implement the hotplug in future we will use the
> passthrough= option to enable VPCI.

So, to summarise, I think the situation is:

 * VCPI is necessry for passthrough on ARM, whether coldplug or
   hotplug.  It's part of the way that PCI-PT works on ARM.

 * Hotplug is not yet implemented.

 * VPCI is not necessary on x86 (evidently, since we don't have it
   there but we do have passthrough).

So when hotplug is added, vpci will need to be turned on when
passthrough=yes is selected.  I don't fully understand the other
possible values for passthrough= but maybe we can defer the question
of whether they apply to ARM ?

I think that means that yes, this should be an internal variable.
Probably in libxl__domain_create_state.  We don't currently arrange to
elide arch-specific state in there, so perhaps it's fine just to
invent a member called `arm_vpci`.

Maybe you could leave a comment somewhere so that if and when PCI PT
hotplug is implemented for ARM, the implementor remembers to wire this
up.

Ian.