On 8/9/2019 4:32 AM, Alex Williamson 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? > > 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. Thanks, Kirti > 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(-) >> >
