> On 21 Jul 2020, at 12:23 am, Stefano Stabellini <sstabell...@kernel.org>
> wrote:
>
> On Fri, 17 Jul 2020, Bertrand Marquis wrote:
>>> On 17 Jul 2020, at 16:06, Jan Beulich <jbeul...@suse.com> wrote:
>>>
>>> On 17.07.2020 15:59, Bertrand Marquis wrote:
>>>>
>>>>
>>>>> On 17 Jul 2020, at 15:19, Jan Beulich <jbeul...@suse.com> wrote:
>>>>>
>>>>> On 17.07.2020 15:14, Bertrand Marquis wrote:
>>>>>>> On 17 Jul 2020, at 10:10, Jan Beulich <jbeul...@suse.com> wrote:
>>>>>>> On 16.07.2020 19:10, Rahul Singh wrote:
>>>>>>>> # Emulated PCI device tree node in libxl:
>>>>>>>>
>>>>>>>> Libxl is creating a virtual PCI device tree node in the device tree to
>>>>>>>> enable the guest OS to discover the virtual PCI during guest boot. We
>>>>>>>> introduced the new config option [vpci="pci_ecam"] for guests. When
>>>>>>>> this config option is enabled in a guest configuration, a PCI device
>>>>>>>> tree node will be created in the guest device tree.
>>>>>>>
>>>>>>> I support Stefano's suggestion for this to be an optional thing, i.e.
>>>>>>> there to be no need for it when there are PCI devices assigned to the
>>>>>>> guest anyway. I also wonder about the pci_ prefix here - isn't
>>>>>>> vpci="ecam" as unambiguous?
>>>>>>
>>>>>> This could be a problem as we need to know that this is required for a
>>>>>> guest upfront so that PCI devices can be assigned after using xl.> >>>
>>>>> I'm afraid I don't understand: When there are no PCI device that get
>>>>> handed to a guest when it gets created, but it is supposed to be able
>>>>> to have some assigned while already running, then we agree the option
>>>>> is needed (afaict). When PCI devices get handed to the guest while it
>>>>> gets constructed, where's the problem to infer this option from the
>>>>> presence of PCI devices in the guest configuration?
>>>>
>>>> If the user wants to use xl pci-attach to attach in runtime a device to a
>>>> guest, this guest must have a VPCI bus (even with no devices).
>>>> If we do not have the vpci parameter in the configuration this use case
>>>> will not work anymore.
>>>
>>> That's what everyone looks to agree with. Yet why is the parameter needed
>>> when there _are_ PCI devices anyway? That's the "optional" that Stefano
>>> was suggesting, aiui.
>>
>> I agree in this case the parameter could be optional and only required if
>> not PCI device is assigned directly in the guest configuration.
>
> Great!
>
> Moreover, we might also be able to get rid of the vpci parameter in
> cases where are no devices assigned at boot time but still we want to
> create a vpci host bridge in domU anyway. In those cases we could use
> the following:
>
> pci = [];
>
> otherwise, worse but it might be easier to implement in xl:
>
> pci = [""];
pci =[] ; is a great idea to avoid new config option to create a device tree
node when there is no device assigned. We will check this and will update the
design spec accodringly.
