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

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Date:   Tue, 20 Aug 2019 11:58:55 +0200
From:   Christophe de Dinechin <christophe.de.dinechin@...il.com>
To:     Parav Pandit <parav@...lanox.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        Jiri Pirko <jiri@...lanox.com>,
        "David S . Miller" <davem@...emloft.net>,
        Kirti Wankhede <kwankhede@...dia.com>,
        Cornelia Huck <cohuck@...hat.com>,
        "kvm\@vger.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "cjia\@nvidia.com" <cjia@...dia.com>,
        "netdev\@vger.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 0/2] Simplify mtty driver and mdev core


Parav Pandit writes:

> + Dave.
>
> Hi Jiri, Dave, Alex, Kirti, Cornelia,
>
> Please provide your feedback on it, how shall we proceed?
>
> Hence, I would like to discuss below options.
>
> Option-1: mdev index
> Introduce an optional mdev index/handle as u32 during mdev create time.
> User passes mdev index/handle as input.
>
> phys_port_name=mIndex=m%u
> mdev_index will be available in sysfs as mdev attribute for udev to name the mdev's netdev.
>
> example mdev create command:
> UUID=$(uuidgen)
> echo $UUID index=10 > /sys/class/net/ens2f0/mdev_supported_types/mlx5_core_mdev/create
> example netdevs:
> repnetdev=ens2f0_m10	/*ens2f0 is parent PF's netdevice */
> mdev_netdev=enm10
>
> Pros:
> 1. mdevctl and any other existing tools are unaffected.
> 2. netdev stack, ovs and other switching platforms are unaffected.
> 3. achieves unique phys_port_name for representor netdev
> 4. achieves unique mdev eth netdev name for the mdev using udev/systemd extension.
> 5. Aligns well with mdev and netdev subsystem and similar to existing sriov bdf's.
>
> Option-2: shorter mdev name
> Extend mdev to have shorter mdev device name in addition to UUID.
> such as 'foo', 'bar'.
> Mdev will continue to have UUID.
> phys_port_name=mdev_name
>
> Pros:
> 1. All same as option-1, except mdevctl needs upgrade for newer usage.
> It is common practice to upgrade iproute2 package along with the kernel.
> Similar practice to be done with mdevctl.
> 2. Newer users of mdevctl who wants to work with non_UUID names, will use newer mdevctl/tools.
> Cons:
> 1. Dual naming scheme of mdev might affect some of the existing tools.
> It's unclear how/if it actually affects.
> mdevctl [2] is very recently developed and can be enhanced for dual naming scheme.
>
> Option-3: mdev uuid alias
> Instead of shorter mdev name or mdev index, have alpha-numeric name alias.
> Alias is an optional mdev sysfs attribute such as 'foo', 'bar'.
> example mdev create command:
> UUID=$(uuidgen)
> echo $UUID alias=foo > /sys/class/net/ens2f0/mdev_supported_types/mlx5_core_mdev/create
> example netdevs:
> examle netdevs:
> repnetdev = ens2f0_mfoo
> mdev_netdev=enmfoo
>
> Pros:
> 1. All same as option-1.
> 2. Doesn't affect existing mdev naming scheme.
> Cons:
> 1. Index scheme of option-1 is better which can number large number of mdevs with fewer characters, simplifying the management tool.

I believe that Alex pointed out another "Cons" to all three options,
which is that it forces user-space to resolve potential race conditions
when creating an index or short name or alias.

Also, what happens if `index=10` is not provided on the command-line?
Does that make the device unusable for your purpose?


--
Cheers,
Christophe de Dinechin (IRC c3d)