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Re: RFC: PCI devices passthrough on Arm design proposal

  • To: Julien Grall <julien@xxxxxxx>
  • From: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
  • Date: Fri, 17 Jul 2020 13:44:15 +0000
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  • Cc: Rahul Singh <Rahul.Singh@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, nd <nd@xxxxxxx>, Julien Grall <julien.grall.oss@xxxxxxxxx>
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  • Thread-topic: RFC: PCI devices passthrough on Arm design proposal

> On 17 Jul 2020, at 15:29, Julien Grall <julien@xxxxxxx> 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 :-)


> Cheers,
> -- 
> Julien Grall



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