linux-kernel - Re: [PATCH] vfio-mdev: support mediated device creation in kernel

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Date:   Thu, 26 Mar 2020 10:16:26 -0700
From:   Yonghyun Hwang <yonghyun@...gle.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Kirti Wankhede <kwankhede@...dia.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Havard Skinnemoen <hskinnemoen@...gle.com>,
        Moritz Fischer <mdf@...nel.org>
Subject: Re: [PATCH] vfio-mdev: support mediated device creation in kernel

On Thu, Mar 26, 2020 at 2:38 AM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Mon, Mar 23, 2020 at 02:33:11PM -0700, Yonghyun Hwang wrote:
> > On Mon, Mar 23, 2020 at 4:14 AM Christoph Hellwig <hch@...radead.org> wrote:
> > >
> > > On Fri, Mar 20, 2020 at 10:59:10AM -0700, Yonghyun Hwang wrote:
> > > > To enable a mediated device, a device driver registers its device to VFIO
> > > > MDev framework. Once the mediated device gets enabled, UUID gets fed onto
> > > > the sysfs attribute, "create", to create the mediated device. This
> > > > additional step happens after boot-up gets complete. If the driver knows
> > > > how many mediated devices need to be created during probing time, the
> > > > additional step becomes cumbersome. This commit implements a new function
> > > > to allow the driver to create a mediated device in kernel.
> > >
> > > Please send this along with your proposed user so that we can understand
> > > the use.  Without that new exports have no chance of going in anyway.
> >
> > My driver is still under development. Do you recommend me to implement
> > an example code for the new exports and re-submit the commit?
>
> Hell no.  The point is that we don't add new APIs unless we have
> actual users (not example code!).  And as Alex mentioned the use case
> is rather questionable anyway, so without a user that actually shows a
> good use case which would remove those doubts it is a complete no-go.

I see. Thank you for your clarification. (I got confused with the use
case.) I hope you can understand email communication results in some
confusion from time to time, which is especially true for kernel
newbie like me.