Re: [RFC PATCH v1 1/4] arm/pci: PCI setup and PCI host bridge discovery within XEN on ARM.

[Julien Grall <julien.grall.oss@xxxxxxxxx>]

On Sat, 25 Jul 2020 at 00:46, Stefano Stabellini <sstabellini@xxxxxxxxxx> wrote:
>
> On Fri, 24 Jul 2020, Julien Grall wrote:
> > On Fri, 24 Jul 2020 at 19:32, Stefano Stabellini <sstabellini@xxxxxxxxxx> 
> > wrote:
> > > > If they are not equal, then I fail to see why it would be useful to 
> > > > have this
> > > > value in Xen.
> > >
> > > I think that's because the domain is actually more convenient to use
> > > because a segment can span multiple PCI host bridges. So my
> > > understanding is that a segment alone is not sufficient to identify a
> > > host bridge. From a software implementation point of view it would be
> > > better to use domains.
> >
> > AFAICT, this would be a matter of one check vs two checks in Xen :).
> > But... looking at Linux, they will also use domain == segment for ACPI
> > (see [1]). So, I think, they still have to use (domain, bus) to do the 
> > lookup.
> >
> > > > In which case, we need to use PHYSDEVOP_pci_mmcfg_reserved so
> > > > Dom0 and Xen can synchronize on the segment number.
> > >
> > > I was hoping we could write down the assumption somewhere that for the
> > > cases we care about domain == segment, and error out if it is not the
> > > case.
> >
> > Given that we have only the domain in hand, how would you enforce that?
> >
> > >From this discussion, it also looks like there is a mismatch between the
> > implementation and the understanding on QEMU devel. So I am a bit
> > concerned that this is not stable and may change in future Linux version.
> >
> > IOW, we are know tying Xen to Linux. So could we implement
> > PHYSDEVOP_pci_mmcfg_reserved *or* introduce a new property that
> > really represent the segment?
>
> I don't think we are tying Xen to Linux. Rob has already said that
> linux,pci-domain is basically a generic device tree property.

My concern is not so much the name of the property, but the definition of it.

AFAICT, from this thread there can be two interpretation:
      - domain == segment
      - domain == (segment, bus)

> And if we
> look at https://www.devicetree.org/open-firmware/bindings/pci/pci2_1.pdf
> "PCI domain" is described and seems to match the Linux definition.
>
> I do think we need to understand the definitions and the differences.

+1

> Reading online [1][2] it looks like a Linux PCI domain matches a "PCI
> Segment Group Number" in PCI Express which is probably why Linux is
> making the assumption that it is making.
>
> So maybe it is OK to use domains == segments, but we need to verify this
> in the specs and also clarify the terminology we use in a doc for our
> own sanity --  I am hoping that Rahul can come up with a good
> explanation on the topic :-)

I am a bit confused.... You were the one arguing that domain ==
(segment, bus) in this thread. So may I ask why the interpretation
wouldn't be valid anymore?

Cheers,

> [1] 
> https://stackoverflow.com/questions/49050847/how-is-pci-segmentdomain-related-to-multiple-host-bridgesor-root-bridges
> [2] https://wiki.osdev.org/PCI_Express