linux-kernel - Re: [PATCH RFC v1 0/3] Introduce vfio-pci-core subsystem

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Date:   Tue, 19 Jan 2021 19:56:10 +0100
From:   Cornelia Huck <cohuck@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Max Gurtovoy <mgurtovoy@...dia.com>, <alex.williamson@...hat.com>,
        <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <liranl@...dia.com>, <oren@...dia.com>, <tzahio@...dia.com>,
        <leonro@...dia.com>, <yarong@...dia.com>, <aviadye@...dia.com>,
        <shahafs@...dia.com>, <artemp@...dia.com>, <kwankhede@...dia.com>,
        <ACurrid@...dia.com>, <gmataev@...dia.com>, <cjia@...dia.com>
Subject: Re: [PATCH RFC v1 0/3] Introduce vfio-pci-core subsystem

On Mon, 18 Jan 2021 14:16:26 -0400
Jason Gunthorpe <jgg@...dia.com> wrote:

> On Mon, Jan 18, 2021 at 05:00:09PM +0100, Cornelia Huck wrote:
> 
> > > You can say that all the HW specific things are in the mlx5_vfio_pci
> > > driver. It is an unusual driver because it must bind to both the PCI
> > > VF with a pci_driver and to the mlx5_core PF using an
> > > auxiliary_driver. This is needed for the object lifetimes to be
> > > correct.  
> > 
> > Hm... I might be confused about the usage of the term 'driver' here.
> > IIUC, there are two drivers, one on the pci bus and one on the
> > auxiliary bus. Is the 'driver' you're talking about here more the
> > module you load (and not a driver in the driver core sense?)  
> 
> Here "driver" would be the common term meaning the code that realizes
> a subsytem for HW - so mlx5_vfio_pci is a VFIO driver because it
> ultimately creates a /dev/vfio* through the vfio subsystem.
> 
> The same way we usually call something like mlx5_en an "ethernet
> driver" not just a "pci driver"
> 
> > Yes, sure. But it also shows that mlx5_vfio_pci aka the device-specific
> > code is rather small in comparison to the common vfio-pci code.
> > Therefore my question whether it will gain more specific changes (that
> > cannot be covered via the auxiliary driver.)  
> 
> I'm not sure what you mean "via the auxiliary driver" - there is only
> one mlx5_vfio_pci, and the non-RFC version with all the migration code
> is fairly big.
> 
> The pci_driver contributes a 'struct pci_device *' and the
> auxiliary_driver contributes a 'struct mlx5_core_dev *'. mlx5_vfio_pci
> fuses them together into a VFIO device. Depending on the VFIO
> callback, it may use an API from the pci_device or from the
> mlx5_core_dev device, or both.

Let's rephrase my question a bit:

This proposal splits the existing vfio-pci driver into a "core"
component and code actually implementing the "driver" part. For mlx5,
an alternative "driver" is introduced that reuses the "core" component
and also hooks into mlx5-specific code parts via the auxiliary device
framework. (IIUC, the plan is to make existing special cases for
devices follow mlx5's lead later.)

I've been thinking of an alternative split: Keep vfio-pci as it is now,
but add an auxiliary device. For mlx5, an auxiliary device_driver can
match to that device and implement mlx5-specific things. From the code
in this RFC, it is not clear to me whether this would be feasible: most
callbacks seem to simply forward to the core component, and that might
be possible to be done by a purely auxiliary device_driver; but this
may or may not work well for additional functionality.

I guess my question is: into which callbacks will the additional
functionality hook? If there's no good way to do what they need to do
without manipulating the vfio-pci calls, my proposal will not work, and
this proposal looks like the better way. But it's hard to tell without
seeing the code, which is why I'm asking :)