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

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Date:   Tue, 21 Sep 2021 13:56:01 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Liu Yi L <yi.l.liu@...el.com>
Cc:     jgg@...dia.com, hch@....de, jasowang@...hat.com, joro@...tes.org,
        jean-philippe@...aro.org, kevin.tian@...el.com, parav@...lanox.com,
        lkml@...ux.net, pbonzini@...hat.com, lushenming@...wei.com,
        eric.auger@...hat.com, corbet@....net, ashok.raj@...el.com,
        yi.l.liu@...ux.intel.com, jun.j.tian@...el.com, hao.wu@...el.com,
        dave.jiang@...el.com, jacob.jun.pan@...ux.intel.com,
        kwankhede@...dia.com, robin.murphy@....com, kvm@...r.kernel.org,
        iommu@...ts.linux-foundation.org, dwmw2@...radead.org,
        linux-kernel@...r.kernel.org, baolu.lu@...ux.intel.com,
        david@...son.dropbear.id.au, nicolinc@...dia.com
Subject: Re: [RFC 02/20] vfio: Add device class for /dev/vfio/devices

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,

Alex