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Re: [PATCH v3 1/2] spapr: Disable legacy virtio devices for pseries-5.0

From: David Gibson
Subject: Re: [PATCH v3 1/2] spapr: Disable legacy virtio devices for pseries-5.0 and later
Date: Thu, 12 Mar 2020 12:14:20 +1100

On Wed, Mar 11, 2020 at 03:11:16AM -0400, Michael S. Tsirkin wrote:
> On Wed, Mar 11, 2020 at 11:58:57AM +1100, David Gibson wrote:
> > Note that several things that I believe are now in the PCIe spec, but
> > really derive more from PC legacy considerations, don't apply at all
> > for PAPR.  e.g. there's no meaningful distinction between integrated
> > and slotted devices, multiple independent host bridges is routine and
> > doesn't require any (virtual) hardware visible domain numbers.
> Domain numbers are a Linux thing, not a PCIe thing. On x86 they come
> from ACPI segment numbers. As such they aren't usually hardware
> visible on x86, they are supplied by firmware.

Oh, ok.  I thought that at least on the standard IO 0xcf8 host bridge
controller the domain number was written into certain registers to
select the relevant root bus.

On POWER the domain numbers are arbitrarily assigned within Linux.
"Hardware" (well, the firmware/hypervisor) uses a different
identifier, called the BUID (generally a large, 64-bit pseudo-address)
in the device tree and hypercalls.

[As an aside, this means the use of domain numbers in libvirt XML is
complete bogosity]

David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!

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