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(-) > >
