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Re: RFC: PCI devices passthrough on Arm design proposal

  • To: Jan Beulich <jbeulich@xxxxxxxx>
  • From: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
  • Date: Fri, 17 Jul 2020 14:34:55 +0000
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  • Cc: Rahul Singh <Rahul.Singh@xxxxxxx>, Julien Grall <julien.grall.oss@xxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, nd <nd@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
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  • Thread-topic: RFC: PCI devices passthrough on Arm design proposal

> 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.


> Jan



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