Re: [PATCH 1/1] s390/pci: expose a PCI device's UID as its index

[Viktor Mihajlovski]

On 4/16/21 7:58 PM, K, Narendra wrote:
> > -----Original Message-----
> > From: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>
> > Sent: Thursday, April 15, 2021 12:55 PM
> > To: Bjorn Helgaas
> > Cc: K, Narendra; Viktor Mihajlovski; Stefan Raspl; Peter Oberparleiter; linux-
> > netdev@xxxxxxxxxxxxxxx; linux-pci@xxxxxxxxxxxxxxx; linux-
> > kernel@xxxxxxxxxxxxxxx; linux-s390@xxxxxxxxxxxxxxx
> > Subject: Re: [PATCH 1/1] s390/pci: expose a PCI device's UID as its index
> >
> >
> > [EXTERNAL EMAIL]
> >
> > On Wed, 2021-04-14 at 15:17 -0500, Bjorn Helgaas wrote:
> > > On Mon, Apr 12, 2021 at 03:59:05PM +0200, Niklas Schnelle wrote:
> > > > On s390 each PCI device has a user-defined ID (UID) exposed under
> > > > /sys/bus/pci/devices/<dev>/uid. This ID was designed to serve as the
> > > > PCI device's primary index and to match the device within Linux to
> > > > the device configured in the hypervisor. To serve as a primary
> > > > identifier the UID must be unique within the Linux instance, this is
> > > > guaranteed by the platform if and only if the UID Uniqueness
> > > > Checking flag is set within the CLP List PCI Functions response.
> > > >
> > > > In this sense the UID serves an analogous function as the SMBIOS
> > > > instance number or ACPI index exposed as the "index" respectively
> > > > "acpi_index" device attributes and used by e.g. systemd to set
> > > > interface names. As s390 does not use and will likely never use ACPI
> > > > nor SMBIOS there is no conflict and we can just expose the UID under the
> > "index"
> > > > attribute whenever UID Uniqueness Checking is active and get
> > > > systemd's interface naming support for free.
> > > >
> > > > Signed-off-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>
> > > > Acked-by: Viktor Mihajlovski <mihajlov@xxxxxxxxxxxxx>
> > >
> > > This seems like a nice solution to me.
> > >
> > > Acked-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> >
> > Thanks! Yes I agree it's a simple solution that also makes sense from a design
> > point. I'll wait for Narendra's opinion of course.
>
> Hi Niklas,
>
> It seems like the UID and the index are not equivalent in their meaning.
Hi Narendra,

the reasoning behind the wish to reuse index is that we think it's
similar in the sense that it provides a stable, firmware-reported
interface identifier.
Some background: s390 is a highly virtualized platform. There's
no traditional bare metal mode of operation, neither for the computer
system nor the I/O subsystem.
The equivalent to a standalone system is a logical partition (LPAR)
which in essence is a kind of virtual machine. LPARs access I/O devices
in a virtualized fashion as well. E.g., for PCI devices the I/O
subsystem is responsible for the management of PCI hardware and
will provide the PCI functions to the LPARs.
This is where UID and function_id come into play. Each PCI function will
carry a function_id attribute which defines it uniquely within the
entire s390 system. If a UID is configured for the function, it must
be unique within an LPAR, but doesn't need to be unique system-wide.
For instance, the admistrator may want to ensure that for every
LPAR the primary ethernet interface has the same identifier, to
allow miigration of Linux instances accross LPARs.
This can't be achieved with a slot-based name.
> The index SMBIOS type 41 device type instance indicates that
>
> 1. The device is an onboard device.
> 2. A specific order of the onboard devices.
>
> The UID described seems to indicate that the PCI device is unique within the Linux instance (sufficient for naming).
> But it does not indicate that PCI device is onboard and any order.
>
> As all devices with UID will be named as eno<UID> (Ethernet onboard), names are not in sync with how each PCI function is exposed on a PCI slot (appears
> closer to SMBIOS type 9 record) as described below.
>
> https://github.com/systemd/systemd/pull/19017
> https://github.com/systemd/systemd/commit/a496a238e8ee66ce25ad13a3f46549b2e2e979fc
>
> In summary, it seems like the eno<UID> names on s390 will be unique names, but may not match the meaning of the index.
>
> Also,
>
> a) Will UID remain unique/same as initially exposed across reboots ?
Yes, the UID is the primary identifier for a PCI function and part of
the LPAR configuration. Even hotplug will not change the UID.
> b) Is the reuse of index related to the 'slots' based naming scheme described below ?
>
> https://github.com/systemd/systemd/pull/19017
> https://github.com/systemd/systemd/commit/a496a238e8ee66ce25ad13a3f46549b2e2e979fc
No, this is independent. The commit above fixes the slot name parsing.
> c) If the slot based naming is fixed with the naming scheme from changes below, any thoughts on why is reuse of index needed ?
>
> https://github.com/systemd/systemd/pull/19017
> https://github.com/systemd/systemd/commit/a496a238e8ee66ce25ad13a3f46549b2e2e979fc
As I wrote above, the slot describes the PCI function at the system
level, whereas the uid/index does it in the context off the LPAR.
>  > With regards,
> Narendra K

--
Kind Regards,
   Viktor