Date:   Fri, 8 Nov 2019 22:48:31 +0000
From:   Parav Pandit <parav@...lanox.com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>
CC:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jiri Pirko <jiri@...nulli.us>,
        David M <david.m.ertman@...el.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Saeed Mahameed <saeedm@...lanox.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "leon@...nel.org" <leon@...nel.org>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        Jiri Pirko <jiri@...lanox.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        Or Gerlitz <gerlitz.or@...il.com>,
        "Jason Wang (jasowang@...hat.com)" <jasowang@...hat.com>
Subject: RE: [PATCH net-next 00/19] Mellanox, mlx5 sub function support

Hi Greg, Jason,

> -----Original Message-----
> From: Alex Williamson <alex.williamson@...hat.com>
> 
> On Fri, 8 Nov 2019 17:05:45 -0400
> Jason Gunthorpe <jgg@...pe.ca> wrote:
> 
> > On Fri, Nov 08, 2019 at 01:34:35PM -0700, Alex Williamson wrote:
> > > On Fri, 8 Nov 2019 16:12:53 -0400
> > > Jason Gunthorpe <jgg@...pe.ca> wrote:
> > >
> > > > On Fri, Nov 08, 2019 at 11:12:38AM -0800, Jakub Kicinski wrote:
> > > > > On Fri, 8 Nov 2019 15:40:22 +0000, Parav Pandit wrote:
> > > > > > > The new intel driver has been having a very similar
> > > > > > > discussion about how to model their 'multi function device'
> > > > > > > ie to bind RDMA and other drivers to a shared PCI function, and I
> think that discussion settled on adding a new bus?
> > > > > > >
> > > > > > > Really these things are all very similar, it would be nice
> > > > > > > to have a clear methodology on how to use the device core if
> > > > > > > a single PCI device is split by software into multiple
> > > > > > > different functional units and attached to different driver instances.
> > > > > > >
> > > > > > > Currently there is alot of hacking in this area.. And a
> > > > > > > consistent scheme might resolve the ugliness with the dma_ops
> wrappers.
> > > > > > >
> > > > > > > We already have the 'mfd' stuff to support splitting
> > > > > > > platform devices, maybe we need to create a 'pci-mfd' to support
> splitting PCI devices?
> > > > > > >
> > > > > > > I'm not really clear how mfd and mdev relate, I always
> > > > > > > thought mdev was strongly linked to vfio.
> > > > > > >
> > > > > >
> > > > > > Mdev at beginning was strongly linked to vfio, but as I
> > > > > > mentioned above it is addressing more use case.
> > > > > >
> > > > > > I observed that discussion, but was not sure of extending mdev
> further.
> > > > > >
> > > > > > One way to do for Intel drivers to do is after series [9].
> > > > > > Where PCI driver says, MDEV_CLASS_ID_I40_FOO
> > > > > > RDMA driver mdev_register_driver(), matches on it and does the
> probe().
> > > > >
> > > > > Yup, FWIW to me the benefit of reusing mdevs for the Intel case vs
> > > > > muddying the purpose of mdevs is not a clear trade off.
> > > >
> > > > IMHO, mdev has amdev_parent_ops structure clearly intended to link
> > > > it to vfio, so using a mdev for something not related to vfio
> > > > seems like a poor choice.
> > >
> > > Unless there's some opposition, I'm intended to queue this for v5.5:
> > >
> > > https://www.spinics.net/lists/kvm/msg199613.html
> > >
> > > mdev has started out as tied to vfio, but at it's core, it's just a
> > > device life cycle infrastructure with callbacks between bus drivers
> > > and vendor devices.  If virtio is on the wrong path with the above
> > > series, please speak up.  Thanks,
> >
> > Well, I think Greg just objected pretty strongly.
> >
> > IMHO it is wrong to turn mdev into some API multiplexor. That is what
> > the driver core already does and AFAIK your bus type is supposed to
> > represent your API contract to your drivers.
> >
> > Since the bus type is ABI, 'mdev' is really all about vfio I guess?
> >
> > Maybe mdev should grow by factoring the special GUID life cycle stuff
> > into a helper library that can make it simpler to build proper API
> > specific bus's using that lifecycle model? ie the virtio I saw
> > proposed should probably be a mdev-virtio bus type providing this new
> > virtio API contract using a 'struct mdev_virtio'?
> 
> I see, the bus:API contract is more clear when we're talking about physical
> buses and physical devices following a hardware specification.
> But if we take PCI for example, each PCI device has it's own internal API that
> operates on the bus API.  PCI bus drivers match devices based on vendor and
> device ID, which defines that internal API, not the bus API.  The bus API is pretty
> thin when we're talking virtual devices and virtual buses though.  The bus "API"
> is essentially that lifecycle management, so I'm having a bit of a hard time
> differentiating this from saying "hey, that PCI bus is nice, but we can't have
> drivers using their own API on the same bus, so can we move the config space,
> reset, hotplug, etc, stuff into helpers and come up with an (ex.) mlx5_bus
> instead?"  Essentially for virtual devices, we're dictating a bus per device type,
> whereas it seemed like a reasonable idea at the time to create a common
> virtual device bus, but maybe it went into the weeds when trying to figure out
> how device drivers match to devices on that bus and actually interact with
> them.
> 
> > I only looked briefly but mdev seems like an unusual way to use the
> > driver core. *generally* I would expect that if a driver wants to
> > provide a foo_device (on a foo bus, providing the foo API contract) it
> > looks very broadly like:
> >
> >   struct foo_device {
> >        struct device dev;
> >        const struct foo_ops *ops;
> >   };
> >   struct my_foo_device {
> >       struct foo_device fdev;
> >   };
> >
> >   foo_device_register(&mydev->fdev);
> >
If I understood Greg's direction on using bus and Jason's suggestion of 'mdev-virtio' example,

User has one of the three use cases as I described in cover letter.
i.e. create a sub device and configure it.
once its configured,
Based on the use case, map it to right bus driver.
1. mdev-vfio (no demux business)
2. virtio (new bus)
3. mlx5_bus (new bus)

We should be creating 3 different buses, instead of mdev bus being de-multiplexer of that?

Hence, depending the device flavour specified, create such device on right bus?

For example,
$ devlink create subdev pci/0000:05:00.0 flavour virtio name foo subdev_id 1
$ devlink create subdev pci/0000:05:00.0 flavour mdev <uuid> subdev_id 2
$ devlink create subdev pci/0000:05:00.0 flavour mlx5 id 1 subdev_id 3

$ devlink subdev pci/0000:05:00.0/<subdev_id> config <params>
$ echo <respective_device_id> <sysfs_path>/bind

Implement power management callbacks also on all above 3 buses?
Abstract out mlx5_bus into more generic virtual bus (vdev bus?) so that multiple vendors can reuse?

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