RE: [PATCH v2 0/2] Simplify mtty driver and mdev core

[Parav Pandit]

> -----Original Message-----
> From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Sent: Tuesday, August 13, 2019 8:23 PM
> To: Parav Pandit <parav@xxxxxxxxxxxx>
> Cc: Kirti Wankhede <kwankhede@xxxxxxxxxx>; kvm@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; cohuck@xxxxxxxxxx; cjia@xxxxxxxxxx
> Subject: Re: [PATCH v2 0/2] Simplify mtty driver and mdev core
> 
> On Tue, 13 Aug 2019 14:40:02 +0000
> Parav Pandit <parav@xxxxxxxxxxxx> wrote:
> 
> > > -----Original Message-----
> > > From: Kirti Wankhede <kwankhede@xxxxxxxxxx>
> > > Sent: Monday, August 12, 2019 5:06 PM
> > > To: Alex Williamson <alex.williamson@xxxxxxxxxx>; Parav Pandit
> > > <parav@xxxxxxxxxxxx>
> > > Cc: kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > > cohuck@xxxxxxxxxx; cjia@xxxxxxxxxx
> > > Subject: Re: [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
> > > > <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.
> > >
> > We need to ask Greg or Linus on the kernel policy on whether an API
> > should exist without in-kernel driver. We don't add such API in
> > netdev, rdma and possibly other subsystem. Where can we find this mdev
> > driver in-tree?
> 
> We probably would not have added the API only for an out of tree driver, but
> we do have a sample driver that uses it, even if it's rather convoluted.  The
> sample driver is showing an example of using the API, which is rather its
> purpose more so than absolutely efficient interrupt handling.  
For showing API use, it doesn't have to convoluted that too in interrupt handling code.
It could be just dev_info(" UUID print..)
But the whole point is to have useful API that non sample driver need to use.
And there is none.
In bigger objective, I wanted to discuss post this cleanup patch, is to expand mdev to have more user friendly device names.

Before we reach there, I should include a patch that eliminates storing UUID itself in the mdev_device.

> Also, let's not
> overstate what this particular API callback provides, it's simply access to the
> uuid of the device, which is a fundamental property of a mediated device.
This fundamental property is available in form of device name already.

> API was added simply to provide data abstraction, allowing the struct
> mdev_device to be opaque to vendor drivers.  Thanks,
> 
I get that part. I prefer to remove the UUID itself from the structure and therefore removing this API makes lot more sense?