lkml.org 
[lkml]   [2019]   [Aug]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v2 0/2] Simplify mtty driver and mdev core
On Thu, 8 Aug 2019 17:02:47 -0600
Alex Williamson <alex.williamson@redhat.com> wrote:

> On Thu, 8 Aug 2019 09:12:53 -0500
> Parav Pandit <parav@mellanox.com> 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(-)
> >
>

\
 
 \ /
  Last update: 2019-08-09 10:08    [W:0.150 / U:3.808 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site