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From: Parav Pandit <parav@mellanox.com>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: Kirti Wankhede <kwankhede@nvidia.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"cohuck@redhat.com" <cohuck@redhat.com>,
	"cjia@nvidia.com" <cjia@nvidia.com>
Subject: RE: [PATCH v2 0/2] Simplify mtty driver and mdev core
Date: Tue, 13 Aug 2019 16:28:53 +0000	[thread overview]
Message-ID: <AM0PR05MB48663579A340E6597B3D01BCD1D20@AM0PR05MB4866.eurprd05.prod.outlook.com> (raw)
In-Reply-To: <20190813085246.1d642ae5@x1.home>



> -----Original Message-----
> From: Alex Williamson <alex.williamson@redhat.com>
> Sent: Tuesday, August 13, 2019 8:23 PM
> To: Parav Pandit <parav@mellanox.com>
> Cc: Kirti Wankhede <kwankhede@nvidia.com>; kvm@vger.kernel.org; linux-
> kernel@vger.kernel.org; cohuck@redhat.com; cjia@nvidia.com
> Subject: Re: [PATCH v2 0/2] Simplify mtty driver and mdev core
> 
> On Tue, 13 Aug 2019 14:40:02 +0000
> Parav Pandit <parav@mellanox.com> wrote:
> 
> > > -----Original Message-----
> > > From: Kirti Wankhede <kwankhede@nvidia.com>
> > > Sent: Monday, August 12, 2019 5:06 PM
> > > To: Alex Williamson <alex.williamson@redhat.com>; Parav Pandit
> > > <parav@mellanox.com>
> > > Cc: kvm@vger.kernel.org; linux-kernel@vger.kernel.org;
> > > cohuck@redhat.com; cjia@nvidia.com
> > > 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@mellanox.com> 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?

  reply	other threads:[~2019-08-13 16:29 UTC|newest]

Thread overview: 74+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-02  6:59 [PATCH 0/2] Simplify mtty driver and mdev core Parav Pandit
2019-08-02  6:59 ` [PATCH 1/2] vfio-mdev/mtty: Simplify interrupt generation Parav Pandit
2019-08-06  8:15   ` Cornelia Huck
2019-08-02  6:59 ` [PATCH 2/2] vfio/mdev: Removed unused and redundant API for mdev name Parav Pandit
2019-08-06  8:29   ` Cornelia Huck
2019-08-06 13:12     ` Parav Pandit
2019-08-06 14:18 ` [PATCH v1 0/2] Simplify mtty driver and mdev core Parav Pandit
2019-08-06 14:18   ` [PATCH v1 1/2] vfio-mdev/mtty: Simplify interrupt generation Parav Pandit
2019-08-06 14:18   ` [PATCH v1 2/2] vfio/mdev: Removed unused and redundant API for mdev UUID Parav Pandit
2019-08-07  9:28     ` Cornelia Huck
2019-08-07 16:33       ` Parav Pandit
2019-08-08  8:29         ` Cornelia Huck
2019-08-08 14:01           ` Parav Pandit
2019-08-08 14:12 ` [PATCH v2 0/2] Simplify mtty driver and mdev core Parav Pandit
2019-08-08 14:12   ` [PATCH v2 1/2] vfio-mdev/mtty: Simplify interrupt generation Parav Pandit
2019-08-13 16:39     ` Christoph Hellwig
2019-08-23 20:48     ` Alex Williamson
2019-08-08 14:12   ` [PATCH v2 2/2] vfio/mdev: Removed unused and redundant API for mdev UUID Parav Pandit
2019-08-13 16:39     ` Christoph Hellwig
2019-08-16 15:22     ` Cornelia Huck
2019-08-08 23:02   ` [PATCH v2 0/2] Simplify mtty driver and mdev core Alex Williamson
2019-08-09  8:07     ` Cornelia Huck
2019-08-12 11:35     ` Kirti Wankhede
2019-08-13 14:40       ` Parav Pandit
2019-08-13 14:52         ` Alex Williamson
2019-08-13 16:28           ` Parav Pandit [this message]
2019-08-13 16:34             ` Cornelia Huck
2019-08-13 17:11             ` Alex Williamson
2019-08-14  5:54               ` Parav Pandit
2019-08-14  8:01                 ` Cornelia Huck
2019-08-14 12:27                   ` Parav Pandit
2019-08-14 13:09                     ` Cornelia Huck
2019-08-14 13:45                       ` Parav Pandit
2019-08-14 14:57                         ` Alex Williamson
2019-08-14 16:21                           ` Parav Pandit
2019-08-20  8:58                             ` Parav Pandit
2019-08-20  9:58                               ` Christophe de Dinechin
2019-08-20 11:25                                 ` Parav Pandit
2019-08-20 16:31                                   ` Cornelia Huck
2019-08-21  2:42                                     ` Parav Pandit
2019-08-20 17:19                               ` Alex Williamson
2019-08-20 17:55                                 ` Cornelia Huck
2019-08-21  3:57                                   ` Parav Pandit
2019-08-21  3:42                                 ` Parav Pandit
2019-08-21  4:20                                   ` Alex Williamson
2019-08-21  4:40                                     ` Parav Pandit
2019-08-21  4:57                                       ` Alex Williamson
2019-08-21  5:01                                         ` Parav Pandit
2019-08-21  5:26                                           ` Alex Williamson
2019-08-21  6:23                                             ` Parav Pandit
2019-08-22  9:29                                               ` Jiri Pirko
2019-08-22  9:42                                                 ` Parav Pandit
2019-08-22  9:58                                                   ` Jiri Pirko
2019-08-22 10:04                                                     ` Parav Pandit
2019-08-22 12:19                                                       ` Jiri Pirko
2019-08-22 13:33                                                         ` Parav Pandit
2019-08-23  8:12                                                           ` Jiri Pirko
2019-08-23  8:14                                                             ` Parav Pandit
2019-08-23 14:28                                                               ` Alex Williamson
2019-08-23 14:53                                                                 ` Parav Pandit
2019-08-23 15:04                                                                   ` Jiri Pirko
2019-08-23 15:52                                                                   ` Alex Williamson
2019-08-23 16:14                                                                     ` Parav Pandit
2019-08-23 17:16                                                                       ` Alex Williamson
2019-08-23 18:00                                                                         ` Parav Pandit
2019-08-23 19:43                                                                           ` Alex Williamson
2019-08-24  3:56                                                                             ` Parav Pandit
2019-08-24  4:45                                                                               ` Parav Pandit
2019-08-24  4:59                                                                               ` Alex Williamson
2019-08-24  5:22                                                                                 ` Parav Pandit
2019-08-13 16:37         ` Christoph Hellwig
2019-08-13 17:40           ` Greg Kroah-Hartman
2019-08-14  5:30             ` Parav Pandit
2019-08-13 14:48     ` Parav Pandit

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