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

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi Ian,

> On 7 Oct 2021, at 17:11, Ian Jackson <iwj@xxxxxxxxxxxxxx> wrote:
> 
> 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.

Sorry for missing this on the v6 serie.

Now you suggest to add a new field arm_vpci in libxl__domain_create_state.

Once we have done that I will need to access this structure to know if I need
to add the DT part and somehow to give it a value depending something which
for now would the number of pcidevs as there will be no user parameter anymore.

I had quite an understanding of the solution using libxl_domain_config and 
changing
The arguments of libxl__arch_domain_init_hw_description and libxl__prepare_dtb
Suggested by Stefano but I am a bit lost in this solution.

The following might be a stupid question but I did not dig a lot in libxl so:
If we add a parameter in the state structure how should we access it ?

Thank
Bertrand



> 
> Ian.