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SubjectRe: [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 <> 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.


> 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(-)

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