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 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?

Cheers,

[1] https://elixir.bootlin.com/linux/latest/source/arch/arm64/kernel/pci.c#L74

Cheers,