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

[Parav Pandit]

Hi Alex,


> -----Original Message-----
> From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Sent: Friday, August 9, 2019 4:33 AM
> To: Parav Pandit <parav@xxxxxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx; kwankhede@xxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; cohuck@xxxxxxxxxx; cjia@xxxxxxxxxx
> Subject: Re: [PATCH v2 0/2] Simplify mtty driver and mdev core
> 
> 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.
> 
This is not the common practice in the kernel to provide exported symbol for every single field of the structure.

> 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.  
Can you please point to the kernel code that accesses the UUID?

> 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?
> 
Yes. it is worthwhile to not keep any dead code in the kernel when there is no in-kernel driver using it.
Did I miss a caller?
Sample driver is setting wrong example of how/when uuid is used.
There has be better example to show how/when/why to use it.
Out of tree driver doesn't qualify API addition to my understanding.
I like to listen to Greg and others for an API inclusion without user as I haven't come across such practice in other subsystems such as nvme, netdev, rdma.

> > 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(-)
> >