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

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Date:   Thu, 26 Mar 2020 02:38:07 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Yonghyun Hwang <yonghyun@...gle.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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 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.