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

[Jiri Pirko <jiri@resnulli.us>]

Fri, Aug 23, 2019 at 04:53:06PM CEST, parav@mellanox.com wrote:
>
>
>> -----Original Message-----
>> From: Alex Williamson <alex.williamson@redhat.com>
>> Sent: Friday, August 23, 2019 7:58 PM
>> To: Parav Pandit <parav@mellanox.com>
>> Cc: Jiri Pirko <jiri@resnulli.us>; 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
>> 
>> On Fri, 23 Aug 2019 08:14:39 +0000
>> Parav Pandit <parav@mellanox.com> wrote:
>> 
>> > Hi Alex,
>> >
>> >
>> > > -----Original Message-----
>> > > From: Jiri Pirko <jiri@resnulli.us>
>> > > Sent: Friday, August 23, 2019 1:42 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
>> > >
>> > > Thu, Aug 22, 2019 at 03:33:30PM CEST, parav@mellanox.com wrote:
>> > > >
>> > > >
>> > > >> -----Original Message-----
>> > > >> From: Jiri Pirko <jiri@resnulli.us>
>> > > >> Sent: Thursday, August 22, 2019 5:50 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
>> > > >>
>> > > >> Thu, Aug 22, 2019 at 12:04:02PM CEST, parav@mellanox.com wrote:
>> > > >> >
>> > > >> >
>> > > >> >> -----Original Message-----
>> > > >> >> From: Jiri Pirko <jiri@resnulli.us>
>> > > >> >> Sent: Thursday, August 22, 2019 3:28 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
>> > > >> >>
>> > > >> >> 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,
>> > > >> >So when mdev creation is initiated, mdev core calculates the
>> > > >> >alias and if there
>> > > >> is any other mdev with same alias exist, it returns -EEXIST error
>> > > >> before progressing further.
>> > > >> >This way user will get to know upfront in event of collision
>> > > >> >before the mdev
>> > > >> device gets created.
>> > > >> >How about that?
>> > > >>
>> > > >> Sounds fine to me. Now the question is how many chars do we want to
>> have.
>> > > >>
>> > > >12 characters from Alex's suggestion similar to git?
>> > >
>> > > Ok.
>> > >
>> >
>> > Can you please confirm this scheme looks good now? I like to get patches
>> started.
>> 
>> My only concern is your comment that in the event of an abbreviated
>> sha1 collision (as exceptionally rare as that might be at 12-chars), we'd fail the
>> device create, while my original suggestion was that vfio-core would add an
>> extra character to the alias.  For non-networking devices, the sha1 is
>> unnecessary, so the extension behavior seems preferred.  The user is only
>> responsible to provide a unique uuid.  Perhaps the failure behavior could be
>> applied based on the mdev device_api.  A module option on mdev to specify the
>> default number of alias chars would also be useful for testing so that we can set
>> it low enough to validate the collision behavior.  Thanks,
>> 
>
>Idea is to have mdev alias as optional.
>Each mdev_parent says whether it wants mdev_core to generate an alias or not.
>So only networking device drivers would set it to true.
>For rest, alias won't be generated, and won't be compared either during creation time.
>User continue to provide only uuid.
>I am tempted to have alias collision detection only within children mdevs of the same parent, but doing so will always mandate to prefix in netdev name.
>And currently we are left with only 3 characters to prefix it, so that may not be good either.
>Hence, I think mdev core wide alias is better with 12 characters.
>
>I do not understand how an extra character reduces collision, if that's what you meant.

Also, that breaks the naming consistency for different creation order.


>Module options are almost not encouraged anymore with other subsystems/drivers.
>
>For testing collision rate, a sample user space script and sample mtty is easy and get us collision count too.
>We shouldn't put that using module option in production kernel.
>I practically have the code ready to play with; Changing 12 to smaller value is easy with module reload.
>
>#define MDEV_ALIAS_LEN 12
>
>> Alex
>> 
>> > > >> >> 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)...
>