Re: RFC: PCI devices passthrough on Arm design proposal

[Julien Grall <julien@xxxxxxx>]

On 17/07/2020 08:41, Oleksandr Andrushchenko wrote:
> > > We need to come up with something similar for dom0less too. It could be
> > > exactly the same thing (a list of BDFs as strings as a device tree
> > > property) or something else if we can come up with a better idea.
> > Fully agree.
> > Maybe a tree topology could allow more possibilities (like giving BAR values) 
> > in the future.
> > > > # 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.
> > > >
> > > > Limitation:
> > > > * Only one PCI device tree node is supported as of now.
> > > I think vpci="pci_ecam" should be optional: if pci=[ "PCI_SPEC_STRING",
> > > ...] is specififed, then vpci="pci_ecam" is implied.
> > >
> > > vpci="pci_ecam" is only useful one day in the future when we want to be
> > > able to emulate other non-ecam host bridges. For now we could even skip
> > > it.
> > This would create a problem if xl is used to add a PCI device as we need the 
> > PCI node to be in the DTB when the guest is created.
> > I agree this is not needed but removing it might create more complexity in the 
> > code.
> I would suggest we have it from day 0 as there are plenty of HW available which 
> is not ECAM.
>
> Having vpci allows other bridges to be supported
So I can understand why you would want to have a driver for non-ECAM 
host PCI controller. However, why would you want to emulate a non-ECAM 
PCI controller to a guest?
Cheers,

--
Julien Grall