netdev - Re: [RFC] PCI: Allow sysfs control over totalvfs

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Message-ID: <BL2PR07MB23061E60A3ABC771F05764C98DF70@BL2PR07MB2306.namprd07.prod.outlook.com>
Date:   Tue, 20 Sep 2016 20:27:24 +0000
From:   "Mintz, Yuval" <Yuval.Mintz@...ium.com>
To:     Jiri Pirko <jiri@...nulli.us>, Yuval Mintz <Yuval.Mintz@...gic.com>
CC:     "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Chickles, Derek" <Derek.Chickles@...ium.com>
Subject: Re: [RFC] PCI: Allow sysfs control over totalvfs

> >Some of the HW capable of SRIOV has resource limitations, where the
> >PF and VFs resources are drawn from a common pool.
> >In some cases, these limitations have to be considered early during
> >chip initialization and can only be changed by tearing down the
> >configuration and re-initializing.
> >As a result, drivers for such HWs sometimes have to make unfavorable
> >compromises where they reserve sufficient resources to accomadate
> >the maximal number of VFs that can be created - at the expanse of
> >resources that could have been used by the PF.
> >
> >If users were able to provide 'hints' regarding the required number
> >of VFs *prior* to driver attachment, then such compromises could be
> >avoided. As we already have sysfs nodes that can be queried for the
> >number of totalvfs, it makes sense to let the user reduce the number
> >of said totalvfs using same infrastrucure.
> >Then, we can have drivers supporting SRIOV take that value into account
> >when deciding how much resources to reserve, allowing the PF to benefit
> >from the difference between the configuration space value and the actual
> >number needed by user.

> One of the motivations for introducing devlink interface was to allow
> user to pass some kind of well defined option parameters or as you call
> it hints to driver module. That would allow to replace module options
> and introduce similar possibility to pre-configure hardware on probe time.
> We plan to use devlink to allow user to change resource allocation for
> mlxsw devices.

Is IOV configuration something you're going to explore in the near
future for mlxsw devices? Or are you merely pointing out that
devlink could provide a superior configuration infrastrucutre and
should be investigated as a better alternative?

> The plan is to allow to pre-create devlink instance before driver module
> is loaded. Then the user will use this placeholder to do the options
> setting. Once the driver module is loaded, it will fetch the options
> from devlink core and process it accordingly.

> I believe this is exactly what you need.

While this sounds far-superior to anything we can do via pci sysfs,
question is whether adding a devlink support for a device is 
a reasonable cost for adding this specific configuration [given
the existing sysfs nodes we already have].
I'm not sufficiently familiar with the infrastrucutre there, and I
wonder whether it will set the bar too high for this sort of
configuration to be used.