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

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Date:   Wed, 22 Sep 2021 09:22:52 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     "Liu, Yi L" <yi.l.liu@...el.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.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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        "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, 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

Jason