linux-kernel - Re: [RFC 03/20] vfio: Add vfio_[un]register

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Message-ID: <20210922141036.5cd46b2b.alex.williamson@redhat.com>
Date:   Wed, 22 Sep 2021 14:10:36 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "hch@....de" <hch@....de>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
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        "lushenming@...wei.com" <lushenming@...wei.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "yi.l.liu@...ux.intel.com" <yi.l.liu@...ux.intel.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>,
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        "david@...son.dropbear.id.au" <david@...son.dropbear.id.au>,
        "nicolinc@...dia.com" <nicolinc@...dia.com>
Subject: Re: [RFC 03/20] vfio: Add vfio_[un]register_device()

On Wed, 22 Sep 2021 09:22:52 -0300
Jason Gunthorpe <jgg@...dia.com> wrote:

> On Wed, Sep 22, 2021 at 09:23:34AM +0000, Tian, Kevin wrote:
> 
> > > Providing an ioctl to bind to a normal VFIO container or group might
> > > allow a reasonable fallback in userspace..  
> > 
> > I didn't get this point though. An error in binding already allows the
> > user to fall back to the group path. Why do we need introduce another
> > ioctl to explicitly bind to container via the nongroup interface?   
> 
> New userspace still needs a fallback path if it hits the 'try and
> fail'. Keeping the device FD open and just using a different ioctl to
> bind to a container/group FD, which new userspace can then obtain as a
> fallback, might be OK.
> 
> Hard to see without going through the qemu parts, so maybe just keep
> it in mind

If we assume that the container/group/device interface is essentially
deprecated once we have iommufd, it doesn't make a lot of sense to me
to tack on a container/device interface just so userspace can avoid
reverting to the fully legacy interface.

But why would we create vfio device interface files at all if they
can't work?  I'm not really on board with creating a try-and-fail
interface for a mechanism that cannot work for a given device.  The
existence of the device interface should indicate that it's supported.
Thanks,

Alex