Re: RFC: PCI devices passthrough on Arm design proposal

[Rahul Singh <Rahul.Singh@xxxxxxx>]

> On 21 Jul 2020, at 12:23 am, Stefano Stabellini <sstabellini@xxxxxxxxxx> 
> wrote:
> 
> On Fri, 17 Jul 2020, Bertrand Marquis wrote:
>>> On 17 Jul 2020, at 16:06, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>> 
>>> On 17.07.2020 15:59, Bertrand Marquis wrote:
>>>> 
>>>> 
>>>>> On 17 Jul 2020, at 15:19, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>>> 
>>>>> On 17.07.2020 15:14, Bertrand Marquis wrote:
>>>>>>> On 17 Jul 2020, at 10:10, Jan Beulich <jbeulich@xxxxxxxx> 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.