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

[Jiri Pirko <jiri@resnulli.us>]

Thu, Aug 22, 2019 at 11:42:13AM CEST, parav@mellanox.com wrote:
>
>
>> -----Original Message-----
>> From: Jiri Pirko <jiri@resnulli.us>
>> Sent: Thursday, August 22, 2019 2:59 PM
>> To: Parav Pandit <parav@mellanox.com>
>> Cc: Alex Williamson <alex.williamson@redhat.com>; 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
>> 
>> 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?
>> 
>Since users sees two devices with same phys_port_name, user should destroy recently created mdev and recreate mdev with different UUID?

Driver should make sure phys port name wont collide, in this case that
it does not provide 2 same attrs for 2 different ports.
Hmm, so the order of creation matters. That is not good.

>> 
>> >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? 
>Not sure if I understood your question correctly, attemping to answer below.
>phys_port_name of devlink port is defined by the first 12 characters of mdev alias.
>> I assume you want to use the same 12(or so) chars, don't you?
>Mdev's netdev will also use the same mdev alias from the sysfs to rename netdev name from ethX to enm<mdev_alias>, where en=Etherenet, m=mdev.
>
>So yes, same 12 characters are use for mdev's netdev and mdev devlink port's phys_port_name.
>
>Is that what are you asking?

Yes. Then you have 3 chars to handle the rest of the name (pci, pf)...