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

[Jiri Pirko <jiri@resnulli.us>]

Wed, Aug 21, 2019 at 08:23:17AM CEST, parav@mellanox.com wrote:
>
>
>> -----Original Message-----
>> From: Alex Williamson <alex.williamson@redhat.com>
>> Sent: Wednesday, August 21, 2019 10:56 AM
>> To: Parav Pandit <parav@mellanox.com>
>> Cc: Jiri Pirko <jiri@mellanox.com>; David S . Miller <davem@davemloft.net>;
>> Kirti Wankhede <kwankhede@nvidia.com>; Cornelia Huck
>> <cohuck@redhat.com>; kvm@vger.kernel.org; linux-kernel@vger.kernel.org;
>> cjia <cjia@nvidia.com>; netdev@vger.kernel.org
>> 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>

What happens in unlikely case of 2 UUIDs collide?


>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.

Does this resolve the identification of devlink port representor? I
assume you want to use the same 12(or so) chars, don't you?