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

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. 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.
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 :-)

[2] https://wiki.osdev.org/PCI_Express



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