Re: [PATCH v1 11/14] xen/arm: Enable the existing x86 virtual PCI support for ARM.

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 16 Sep 2021, Rahul Singh wrote:
> Hi Stefano,
> 
> > On 10 Sep 2021, at 1:26 am, Stefano Stabellini <sstabellini@xxxxxxxxxx> 
> > wrote:
> > 
> > On Thu, 19 Aug 2021, Rahul Singh wrote:
> >> The existing VPCI support available for X86 is adapted for Arm.
> >> When the device is added to XEN via the hyper call
> >> “PHYSDEVOP_pci_device_add”, VPCI handler for the config space
> >> access is added to the Xen to emulate the PCI devices config space.
> > 
> > This is done just for device discovery, right?
> > 
> > Although it is currently not implemented (and I am not asking to
> > implement it now, I am only trying to understand the architecture), it
> > would be possible to discover all PCI devices just by walking down the
> > PCI hierarchy by ourselves in Xen (no Dom0 interactions) given that we
> > have an ECAM driver.
> > 
> > I take that would be the way to implement PCI support for Dom0less?
> 
> It is not possible to discover PCI devices in XEN if enumeration is not done 
> before XEN boot.
> If boot firmware did the enumeration,  XEN will discover the PCI device.

OK, but if the boot firmware does the enumeration, how will Xen discover
the PCI device exactly? Will Xen discover it because the PCI device will
be present in device tree explicitly (will have its own device tree
node)? Or will Xen discover it by walking the PCI bus?