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From:   Parav Pandit <parav@mellanox.com>
To:     Alex Williamson <alex.williamson@redhat.com>
CC:     "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "kwankhede@nvidia.com" <kwankhede@nvidia.com>,
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        "cohuck@redhat.com" <cohuck@redhat.com>,
        "cjia@nvidia.com" <cjia@nvidia.com>
Subject: RE: [PATCH v2 0/2] Simplify mtty driver and mdev core
Thread-Topic: [PATCH v2 0/2] Simplify mtty driver and mdev core
Thread-Index: AQHVTfNxjgfwJJG2ZUiuOAmKCwQvf6bx3uKAgAdPQrA=
Date:   Tue, 13 Aug 2019 14:48:23 +0000
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Hi Alex,


> -----Original Message-----
> From: Alex Williamson <alex.williamson@redhat.com>
> Sent: Friday, August 9, 2019 4:33 AM
> To: Parav Pandit <parav@mellanox.com>
> Cc: kvm@vger.kernel.org; kwankhede@nvidia.com; linux-
> kernel@vger.kernel.org; cohuck@redhat.com; cjia@nvidia.com
> Subject: Re: [PATCH v2 0/2] Simplify mtty driver and mdev core
>=20
> On Thu,  8 Aug 2019 09:12:53 -0500
> Parav Pandit <parav@mellanox.com> wrote:
>=20
> > 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 uui=
d.
> > 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.
>=20
> Is the interrupt handling efficiency of this particular sample driver rea=
lly
> relevant, or is its purpose more to illustrate the API and provide a proo=
f of
> concept?  If we go to the trouble to optimize the sample driver and remov=
e this
> interface from the API, what do we lose?
>=20
> This interface was added via commit:
>=20
> 99e3123e3d72 vfio-mdev: Make mdev_device private and abstract interfaces
>=20
> 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-tre=
e mdev
> vendor drivers more supportable; the object is considered opaque and acce=
ss is
> provided via an API rather than through direct structure fields.
>=20
This is not the common practice in the kernel to provide exported symbol fo=
r every single field of the structure.

> I believe that the NVIDIA GRID mdev driver does make use of this interfac=
e and
> it's likely included in the sample driver specifically so that there is a=
n 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 the=
re
> are some efficiencies to be gained in having direct access to the UUID fo=
r a
> device when interacting with userspace, rather than repeatedly parsing it=
 from
> a device name. =20
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 UU=
ID
> for it's name, as tools like libvirt and mdevctl expect, is it really wor=
thwhile to
> remove such a trivial API?
>=20
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.
>=20
> s/no production driver/no in-kernel production driver/
>=20
> I'd be interested to hear how the NVIDIA folks make use of this API inter=
face.
> Thanks,
>=20
> Alex
>=20
> > ---
> > 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(-)
> >