linux-kernel - RE: [RFC 02/20] vfio: Add device class for /dev/vfio/devices

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Date:   Wed, 22 Sep 2021 00:56:17 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>
CC:     "jgg@...dia.com" <jgg@...dia.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>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "lkml@...ux.net" <lkml@...ux.net>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "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>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "david@...son.dropbear.id.au" <david@...son.dropbear.id.au>,
        "nicolinc@...dia.com" <nicolinc@...dia.com>
Subject: RE: [RFC 02/20] vfio: Add device class for /dev/vfio/devices

> From: Alex Williamson <alex.williamson@...hat.com>
> Sent: Wednesday, September 22, 2021 3:56 AM
> 
> On Sun, 19 Sep 2021 14:38:30 +0800
> Liu Yi L <yi.l.liu@...el.com> wrote:
> 
> > This patch introduces a new interface (/dev/vfio/devices/$DEVICE) for
> > userspace to directly open a vfio device w/o relying on container/group
> > (/dev/vfio/$GROUP). Anything related to group is now hidden behind
> > iommufd (more specifically in iommu core by this RFC) in a device-centric
> > manner.
> >
> > In case a device is exposed in both legacy and new interfaces (see next
> > patch for how to decide it), this patch also ensures that when the device
> > is already opened via one interface then the other one must be blocked.
> >
> > Signed-off-by: Liu Yi L <yi.l.liu@...el.com>
> > ---
> >  drivers/vfio/vfio.c  | 228 +++++++++++++++++++++++++++++++++++++++----
> >  include/linux/vfio.h |   2 +
> >  2 files changed, 213 insertions(+), 17 deletions(-)
> >
> > diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> > index 02cc51ce6891..84436d7abedd 100644
> > --- a/drivers/vfio/vfio.c
> > +++ b/drivers/vfio/vfio.c
> ...
> > @@ -2295,6 +2436,52 @@ static struct miscdevice vfio_dev = {
> >  	.mode = S_IRUGO | S_IWUGO,
> >  };
> >
> > +static char *vfio_device_devnode(struct device *dev, umode_t *mode)
> > +{
> > +	return kasprintf(GFP_KERNEL, "vfio/devices/%s", dev_name(dev));
> > +}
> 
> dev_name() doesn't provide us with any uniqueness guarantees, so this
> could potentially generate naming conflicts.  The similar scheme for
> devices within an iommu group appends an instance number if a conflict
> occurs, but that solution doesn't work here where the name isn't just a
> link to the actual device.  Devices within an iommu group are also
> likely associated within a bus_type, so the potential for conflict is
> pretty negligible, that's not the case as vfio is adopted for new
> device types.  Thanks,
> 

This is also our concern. Thanks for confirming it. Appreciate if you
can help think out some better alternative to deal with it.

Thanks
Kevin