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

[Cornelia Huck]

On Thu, 8 Aug 2019 17:02:47 -0600
Alex Williamson <alex.williamson@xxxxxxxxxx> 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?

Not sure how useful the sample driver is as a template; blindly copying
their interrupt handling is probably not a good idea.

> 
> 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.  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?

Ripping out the uuid is a bad idea, I agree. The device name simply is
no good replacement for that.

If there's a good use case for using the uuid in a vendor driver, let's
keep the accessor. But then we probably should either leave the sample
driver alone, or add a more compelling use of the api there.

> 
> > 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,
> 
> Alex
> 
> > ---
> > Changelog:
> > v1->v2:
> >  - Corrected email of Kirti
> >  - Updated cover letter commit log to address comment from Cornelia
> >  - Added Reviewed-by tag
> > v0->v1:
> >  - Updated commit log
> > 
> > Parav Pandit (2):
> >   vfio-mdev/mtty: Simplify interrupt generation
> >   vfio/mdev: Removed unused and redundant API for mdev UUID
> > 
> >  drivers/vfio/mdev/mdev_core.c |  6 ------
> >  include/linux/mdev.h          |  1 -
> >  samples/vfio-mdev/mtty.c      | 39 +++++++----------------------------
> >  3 files changed, 8 insertions(+), 38 deletions(-)
> >   
>