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

[Parav Pandit]

> -----Original Message-----
> From: Kirti Wankhede <kwankhede@xxxxxxxxxx>
> Sent: Monday, August 12, 2019 5:06 PM
> To: Alex Williamson <alex.williamson@xxxxxxxxxx>; Parav Pandit
> <parav@xxxxxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; cohuck@xxxxxxxxxx;
> cjia@xxxxxxxxxx
> Subject: Re: [PATCH v2 0/2] Simplify mtty driver and mdev core
> 
> 
> 
> On 8/9/2019 4:32 AM, Alex Williamson wrote:
> > On Thu,  8 Aug 2019 09:12:53 -0500
> > Parav Pandit <parav@xxxxxxxxxxxx> wrote:
> >
> >> Currently mtty sample driver uses mdev state and UUID in convoluated
> >> way to generate an interrupt.
> >> It uses several translations from mdev_state to mdev_device to mdev uuid.
> >> After which it does linear search of long uuid comparision to find
> >> out mdev_state in mtty_trigger_interrupt().
> >> mdev_state is already available while generating interrupt from which
> >> all such translations are done to reach back to mdev_state.
> >>
> >> This translations are done during interrupt generation path.
> >> This is unnecessary and reduandant.
> >
> > Is the interrupt handling efficiency of this particular sample driver
> > really relevant, or is its purpose more to illustrate the API and
> > provide a proof of concept?  If we go to the trouble to optimize the
> > sample driver and remove this interface from the API, what do we lose?
> >
> > This interface was added via commit:
> >
> > 99e3123e3d72 vfio-mdev: Make mdev_device private and abstract
> > interfaces
> >
> > Where the goal was to create a more formal interface and abstract
> > driver access to the struct mdev_device.  In part this served to make
> > out-of-tree mdev vendor drivers more supportable; the object is
> > considered opaque and access is provided via an API rather than
> > through direct structure fields.
> >
> > I believe that the NVIDIA GRID mdev driver does make use of this
> > interface and it's likely included in the sample driver specifically
> > so that there is an in-kernel user for it (ie. specifically to avoid
> > it being removed so casually).  An interesting feature of the NVIDIA
> > mdev driver is that I believe it has portions that run in userspace.
> > As we know, mdevs are named with a UUID, so I can imagine there are
> > some efficiencies to be gained in having direct access to the UUID for
> > a device when interacting with userspace, rather than repeatedly
> > parsing it from a device name.
> 
> That's right.
> 
> >  Is that really something we want to make more difficult in order to
> > optimize a sample driver?  Knowing that an mdev device uses a UUID for
> > it's name, as tools like libvirt and mdevctl expect, is it really
> > worthwhile to remove such a trivial API?
> >
> >> Hence,
> >> Patch-1 simplifies mtty sample driver to directly use mdev_state.
> >>
> >> Patch-2, Since no production driver uses mdev_uuid(), simplifies and
> >> removes redandant mdev_uuid() exported symbol.
> >
> > s/no production driver/no in-kernel production driver/
> >
> > I'd be interested to hear how the NVIDIA folks make use of this API
> > interface.  Thanks,
> >
> 
> Yes, NVIDIA mdev driver do use this interface. I don't agree on removing
> mdev_uuid() interface.
> 
We need to ask Greg or Linus on the kernel policy on whether an API should exist without in-kernel driver.
We don't add such API in netdev, rdma and possibly other subsystem.
Where can we find this mdev driver in-tree?