Re: [Qemu-devel] [PATCH] spapr/pci: populate PCI DT in reverse order

[Thomas Huth <thuth@redhat.com>]

On 22.02.2017 11:56, Greg Kurz wrote:
> From: Greg Kurz <gkurz@linux.vnet.ibm.com>
[...]
> This patch reverts to the historical SLOF ordering by walking PCI devices
> in reverse order. This reconciles pseries with x86 machine types behavior.
> It is expected to make things easier when porting existing applications to
> power.
[...]
> This patch was posted and already discussed during 2.5 development:
> 
> http://patchwork.ozlabs.org/patch/549925/
> 
> The "consensus" at the time was that guests should not rely on device
> ordering (i.e. use persistent naming instead).
> 
> I got recently contacted by OpenStack people who had several complaints
> about the reverse ordering of PCI devices in pseries: different behavior
> between ppc64 and x86, lots of time spent in debugging when porting
> applications from x86 to ppc64 before realizing that it is caused by the
> reverse ordering, necessity to carry hacky workarounds...
> 
> One strong argument against handling this properly with persistent naming
> is that it requires systemd/udev. This option is considered as painful
> with CirrOS, which aims at remaining as minimal as possible and is widely
> used in the OpenStack ecosystem.
> 
> Would you re-consider your position and apply this patch ?

+1 for applying the patch.

During the past months, I've also run one or two times into issues with
the reversed ordering... fortunately, I was able to work around them (or
fix other bugs triggered by this), but I think it would be better to
return the the ascending order again to avoid further future problems.

 Thomas