Re: [PATCH] xen/arm: do not try to add pci-domain for disabled devices

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 27 Oct 2021, Julien Grall wrote:
> Hi Oleksandr,
> 
> On 27/10/2021 09:37, Oleksandr Andrushchenko wrote:
> > From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> > 
> > If a PCI host bridge device is present in the device tree, but is
> > disabled, then its PCI host bridge driver was not instantiated.
> > This results in the following panic during Xen start:
> > 
> > (XEN) Device tree generation failed (-22).
> 
> It would good to clarify in the commit message where the error is coming from.
> I think this is from pci_get_host_bridge_segment().
> 
> > (XEN)
> > (XEN) ****************************************
> > (XEN) Panic on CPU 0:
> > (XEN) Could not set up DOM0 guest OS
> > (XEN) ****************************************
> > 
> > Fix this by not adding linux,pci-domain property for hwdom if it is
> > neither available nor device enabled.
> From my reading of the binding [1], the property should be present in all the
> hostbridges if one specify it. IOW, I think the property should also be added
> for hostbridges that are not available.

Just wanted to say that I think you are right:

"""
It is required to either not set this property at all or set it for all
host bridges in the system, otherwise potentially conflicting domain numbers
may be assigned to root buses behind different host bridges.  The domain
number for each host bridge in the system must be unique.
"""

and I am ready to believe device trees with disabled bridges might have
(incorrectly) ignored the rule.


> AFAICT, Linux will ignore hostbridge that are not available. But it feels to
> me we are twisting the rule. So, could we consider to allocate an unused
> number?

I think that would be fine but it doesn't look easy from the current Xen
code point of view because the allocation depends on the Xen driver,
which we don't have. But I'll let others comment on it. Otherwise
skipping the disabled host bridge node for Dom0 sounds OK.