Re: udev virtio by-path naming

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Mon, 20 Feb 2017 15:34:49 +0100
Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com> wrote:

> Hi,
> 
> with systemd > v229 all virtio block devices will receive persistent
> device names in the format /dev/disk-by/virtio-pci-<busid>, the
> last component being the udev built-in path_id.
> 
> This naming introduces some issues.
> 
> First and obvious, there are virtio implementations not based
> on PCI, like virtio-ccw (currently only on s390) and virtio-mmio
> (for which I can't speak). This results in persistent names like
> /dev/disk-by/virtio-pci-0.0.0001, where the bus id is a CCW id.
> One seemingly obvious remedy would be to make the path_id return
> virtio-ccw-<busid> or more generally virtio-<subsystem>-<busid>,
> both easily done with small patches to systemd-udev.
> 
> But then, I find this naming scheme somewhat weird.
> A virtio disk shows up as a regular PCI function on the PCI
> bus side by side with other (non-virtio) devices. The naming otoh
> suggests that virtio-pci is a subsystem of its own, which is simply
> incorrect from a by-path perspective.

From the ccw perspective, this is quite similar: The virtio proxy
device shows up on the ccw bus, just like e.g. a dasd device shows up
on the ccw bus.

> 
> Using just the plain PCI path id is actually sufficient to identify
> a virtio disk by its path. This would be in line with virtio
> network interface path names which use the plain PCI naming.

Same for ccw: The id on the ccw bus (devno) is already unique and
persistent.

> 
> One could argue about back-level compatibility, but virtio by-path
> naming has changed multiple times. We have seen virtio-pci-virtio<n>
> (not predictable), pci-<busid> and virtio-pci-<busid> already. It
> might be a good time now to settle on a common approach for all
> virtio types.
> 
> For the reasons above, I'd vote for <subsystem>-<busid>, which
> would work for PCI and CCW, not sure about ARM MMIO though.
> Opinions?

I'm not sure whether there is any reason to make virtio special,
although this depends upon what virtio-mmio looks like in the Linux
device model (any arm folks here?)

In the end, I'd be happy with any naming scheme that does not include
'pci' for non-pci devices.