RE: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Wednesday, September 22, 2021 1:41 AM
> 
> On Sun, Sep 19, 2021 at 02:38:38PM +0800, Liu Yi L wrote:
> > After a device is bound to the iommufd, userspace can use this interface
> > to query the underlying iommu capability and format info for this device.
> > Based on this information the user then creates I/O address space in a
> > compatible format with the to-be-attached devices.
> >
> > Device cookie which is registered at binding time is used to mark the
> > device which is being queried here.
> >
> > Signed-off-by: Liu Yi L <yi.l.liu@xxxxxxxxx>
> >  drivers/iommu/iommufd/iommufd.c | 68
> +++++++++++++++++++++++++++++++++
> >  include/uapi/linux/iommu.h      | 49 ++++++++++++++++++++++++
> >  2 files changed, 117 insertions(+)
> >
> > diff --git a/drivers/iommu/iommufd/iommufd.c
> b/drivers/iommu/iommufd/iommufd.c
> > index e16ca21e4534..641f199f2d41 100644
> > +++ b/drivers/iommu/iommufd/iommufd.c
> > @@ -117,6 +117,71 @@ static int iommufd_fops_release(struct inode
> *inode, struct file *filep)
> >  	return 0;
> >  }
> >
> > +static struct device *
> > +iommu_find_device_from_cookie(struct iommufd_ctx *ictx, u64
> dev_cookie)
> > +{
> 
> We have an xarray ID for the device, why are we allowing userspace to
> use the dev_cookie as input?
> 
> Userspace should always pass in the ID. The only place dev_cookie
> should appear is if the kernel generates an event back to
> userspace. Then the kernel should return both the ID and the
> dev_cookie in the event to allow userspace to correlate it.
> 

A little background.

In earlier design proposal we discussed two options. One is to return
an kernel-allocated ID (label) to userspace. The other is to have user
register a cookie and use it in iommufd uAPI. At that time the two
options were discussed exclusively and the cookie one is preferred.

Now you instead recommended a mixed option. We can follow it for
sure if nobody objects.

Thanks
Kevin