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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 28 Oct 2021, Julien Grall wrote:
> Hi Stefano,
> 
> On 28/10/2021 00:57, Stefano Stabellini wrote:
> > 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.
> 
> Looking at linux/arch/arm64/boot/dts/, there are a few Device-Tree that
> contain the property "linux,pci-domain". All of them seems to also add it for
> disabled hostbridges.
> 
> However, I am under the impression that it is more common to have a
> Devide-Tree without any property "linux,pci-domain". When PCI support is
> enabled, Xen will generate the domain ID for the hostbridge and write it to
> the DT.
> 
> This doesn't work for disabled hostbridge and I think this is Oleksandr's
> problem. @Oleksandr can you confirm it?
> 
> > 
> > 
> > > 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.
> So what matters is Xen doesn't make things worse. We have two cases to care:
>   1) Xen only has drivers for a part of the hostbriges
>   2) Some hostbriges are disabled
> 
> Case 1) will definitely generate a DT that will make Linux unhappy if we have
> don't add the property consistently.

Good point!