RE: [PATCH v2 0/2] Simplify mtty driver and mdev core

[Parav Pandit]

> -----Original Message-----
> From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Sent: Wednesday, August 21, 2019 10:56 AM
> To: Parav Pandit <parav@xxxxxxxxxxxx>
> Cc: Jiri Pirko <jiri@xxxxxxxxxxxx>; David S . Miller <davem@xxxxxxxxxxxxx>;
> Kirti Wankhede <kwankhede@xxxxxxxxxx>; Cornelia Huck
> <cohuck@xxxxxxxxxx>; kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> cjia <cjia@xxxxxxxxxx>; netdev@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH v2 0/2] Simplify mtty driver and mdev core
> 
> > > > > Just an example of the alias, not proposing how it's set.  In
> > > > > fact, proposing that the user does not set it, mdev-core
> > > > > provides one
> > > automatically.
> > > > >
> > > > > > > Since there seems to be some prefix overhead, as I ask about
> > > > > > > above in how many characters we actually have to work with
> > > > > > > in IFNAMESZ, maybe we start with 8 characters (matching your
> > > > > > > "index" namespace) and expand as necessary for disambiguation.
> > > > > > > If we can eliminate overhead in IFNAMESZ, let's start with 12.
> > > > > > > Thanks,
> > > > > > >
> > > > > > If user is going to choose the alias, why does it have to be limited to
> sha1?
> > > > > > Or you just told it as an example?
> > > > > >
> > > > > > It can be an alpha-numeric string.
> > > > >
> > > > > No, I'm proposing a different solution where mdev-core creates
> > > > > an alias based on an abbreviated sha1.  The user does not provide the
> alias.
> > > > >
> > > > > > Instead of mdev imposing number of characters on the alias, it
> > > > > > should be best
> > > > > left to the user.
> > > > > > Because in future if netdev improves on the naming scheme,
> > > > > > mdev will be
> > > > > limiting it, which is not right.
> > > > > > So not restricting alias size seems right to me.
> > > > > > User configuring mdev for networking devices in a given kernel
> > > > > > knows what
> > > > > user is doing.
> > > > > > So user can choose alias name size as it finds suitable.
> > > > >
> > > > > That's not what I'm proposing, please read again.  Thanks,
> > > >
> > > > I understood your point. But mdev doesn't know how user is going
> > > > to use
> > > udev/systemd to name the netdev.
> > > > So even if mdev chose to pick 12 characters, it could result in collision.
> > > > Hence the proposal to provide the alias by the user, as user know
> > > > the best
> > > policy for its use case in the environment its using.
> > > > So 12 character sha1 method will still work by user.
> > >
> > > Haven't you already provided examples where certain drivers or
> > > subsystems have unique netdev prefixes?  If mdev provides a unique
> > > alias within the subsystem, couldn't we simply define a netdev
> > > prefix for the mdev subsystem and avoid all other collisions?  I'm
> > > not in favor of the user providing both a uuid and an
> > > alias/instance.  Thanks,
> > >
> > For a given prefix, say ens2f0, can two UUID->sha1 first 9 characters have
> collision?
> 
> I think it would be a mistake to waste so many chars on a prefix, but 9
> characters of sha1 likely wouldn't have a collision before we have 10s of
> thousands of devices.  Thanks,
> 
> Alex

Jiri, Dave,
Are you ok with it for devlink/netdev part?
Mdev core will create an alias from a UUID.

This will be supplied during devlink port attr set such as,

devlink_port_attrs_mdev_set(struct devlink_port *port, const char *mdev_alias);

This alias is used to generate representor netdev's phys_port_name.
This alias from the mdev device's sysfs will be used by the udev/systemd to generate predicable netdev's name.
Example: enm<mdev_alias_first_12_chars>
I took Ethernet mdev as an example.
New prefix 'm' stands for mediated device.
Remaining 12 characters are first 12 chars of the mdev alias.