Re: RFC: PCI devices passthrough on Arm design proposal

[Julien Grall <julien@xen.org>]

On 17/07/2020 14:44, Bertrand Marquis wrote:
> 
> 
>> On 17 Jul 2020, at 15:29, Julien Grall <julien@xen.org> wrote:
>>
>>
>>
>> On 17/07/2020 14:22, Bertrand Marquis 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.
>>>>>
>>>>> A new area has been reserved in the arm guest physical map at which
>>>>> the VPCI bus is declared in the device tree (reg and ranges
>>>>> parameters of the node). A trap handler for the PCI ECAM access from
>>>>> guest has been registered at the defined address and redirects
>>>>> requests to the VPCI driver in Xen.
>>>>
>>>> Can't you deduce the requirement of such DT node based on the presence
>>>> of a 'pci=' option in the same config file?
>>>>
>>>> Also I wouldn't discard that in the future you might want to use
>>>> different emulators for different devices, so it might be helpful to
>>>> introduce something like:
>>>>
>>>> pci = [ '08:00.0,backend=vpci', '09:00.0,backend=xenpt', '0a:00.0,backend=qemu', ... ]
>>
>> I like this idea :).
>>
>>>>
>>>> For the time being Arm will require backend=vpci for all the passed
>>>> through devices, but I wouldn't rule out this changing in the future.
>>> We need it for the case where no device is declared in the config file and the user
>>> wants to add devices using xl later. In this case we must have the DT node for it
>>> to work.
>>
>> Are you suggesting that you plan to implement PCI hotplug?
> 
> No this is not in the current plan but we should not prevent this to be supported some day :-)

I agree that we don't want to prevent extension. But I fail to see why 
this would be an issue if we don't introduce the option "vcpi" today.

Cheers,

-- 
Julien Grall