Re: RFC: PCI devices passthrough on Arm design proposal

[Julien Grall <julien@xxxxxxx>]

On 17/07/2020 14:44, Bertrand Marquis wrote:
> > 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 :-)
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