Re: [PATCH 0/1] Use of /sys/bus/pci/devices/…/index for non-SMBIOS platforms

[Leon Romanovsky <leon@kernel.org>]

On Tue, Apr 13, 2021 at 08:57:19AM +0200, Niklas Schnelle wrote:
> On Tue, 2021-04-13 at 08:39 +0300, Leon Romanovsky wrote:
> > On Mon, Apr 12, 2021 at 03:59:04PM +0200, Niklas Schnelle wrote:
> > > Hi Narendra, Hi All,
> > > 
> > > According to Documentation/ABI/testing/sysfs-bus-pci you are responsible
> > > for the index device attribute that is used by systemd to create network
> > > interface names.
> > > 
> > > Now we would like to reuse this attribute for firmware provided PCI
> > > device index numbers on the s390 architecture which doesn't have
> > > SMBIOS/DMI nor ACPI. All code changes are within our architecture
> > > specific code but I'd like to get some Acks for this reuse. I've sent an
> > > RFC version of this patch on 15th of March with the subject:
> > > 
> > >    s390/pci: expose a PCI device's UID as its index
> > > 
> > > but got no response. Would it be okay to re-use this attribute for
> > > essentially the same purpose but with index numbers provided by
> > > a different platform mechanism? I think this would be cleaner than
> > > further proliferation of /sys/bus/pci/devices/<dev>/xyz_index
> > > attributes and allows re-use of the existing userspace infrastructure.
> > 
> > I'm missing an explanation that this change is safe for systemd and
> > they don't have some hard-coded assumption about the meaning of existing
> > index on s390.
> > 
> > Thanks
> 
> 
> Sure, good point. So first off yes this change does create new index
> based names also on existing systemd versions, this is known and
> intended and we'll certainly closely collaborate with any distributions
> wishing to backport this change.
> 
> As for being otherwise safe or having unintended consequences, Viktor
> (see R-b) and I recently got the following PR merged in that exact area
> of systemd to fix how hotplug slot derived interface names are
> generated:
> https://github.com/systemd/systemd/pull/19017
> In working on that we did also analyse the use of the index attribute
> for hidden assumptions and tested with this attribute added. Arguably,
> as the nature of that PR shows we haven't had a perfect track record of
> keeping this monitored but will in the future as PCI based NICs become
> increasingly important for our platform. We also have special NIC
> naming logic in the same area for our channel based platform specific
> NICs which was also contributed by Viktor.

Thanks, this PR is exciting to read, very warm words were said about
kernel developers :). Can you please summarize that will be the breakage
in old systemd if this index will be overloaded?

Thanks

> 
> Thanks,
> Niklas
>