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Date:   Thu, 7 Oct 2021 08:35:03 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     David Gibson <david@gibson.dropbear.id.au>
Cc:     Liu Yi L <yi.l.liu@intel.com>, alex.williamson@redhat.com,
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        linux-kernel@vger.kernel.org, baolu.lu@linux.intel.com,
        nicolinc@nvidia.com
Subject: Re: [RFC 07/20] iommu/iommufd: Add iommufd_[un]bind_device()
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On Thu, Oct 07, 2021 at 12:23:13PM +1100, David Gibson wrote:
> On Fri, Oct 01, 2021 at 09:43:22AM -0300, Jason Gunthorpe wrote:
> > On Thu, Sep 30, 2021 at 01:10:29PM +1000, David Gibson wrote:
> > > On Wed, Sep 29, 2021 at 09:24:57AM -0300, Jason Gunthorpe wrote:
> > > > On Wed, Sep 29, 2021 at 03:25:54PM +1000, David Gibson wrote:
> > > > 
> > > > > > +struct iommufd_device {
> > > > > > +	unsigned int id;
> > > > > > +	struct iommufd_ctx *ictx;
> > > > > > +	struct device *dev; /* always be the physical device */
> > > > > > +	u64 dev_cookie;
> > > > > 
> > > > > Why do you need both an 'id' and a 'dev_cookie'?  Since they're both
> > > > > unique, couldn't you just use the cookie directly as the index into
> > > > > the xarray?
> > > > 
> > > > ID is the kernel value in the xarray - xarray is much more efficient &
> > > > safe with small kernel controlled values.
> > > > 
> > > > dev_cookie is a user assigned value that may not be unique. It's
> > > > purpose is to allow userspace to receive and event and go back to its
> > > > structure. Most likely userspace will store a pointer here, but it is
> > > > also possible userspace could not use it.
> > > > 
> > > > It is a pretty normal pattern
> > > 
> > > Hm, ok.  Could you point me at an example?
> > 
> > For instance user_data vs fd in io_uring
> 
> Ok, but one of those is an fd, which is an existing type of handle.
> Here we're introducing two different unique handles that aren't an
> existing kernel concept.

I'm not sure how that matters, the kernel has many handles - and we
get to make more of them.. Look at xarray/idr users in the kernel, many of
those are making userspace handles.

> That said... is there any strong reason why user_data needs to be
> unique?  I can imagine userspace applications where you don't care
> which device the notification is coming from - or at least don't care
> down to the same granularity that /dev/iommu is using.  In which case
> having the kernel provided unique handle and the
> not-necessarily-unique user_data would make perfect sense.

I don't think the user_data 64 bit value should be unique, it is just
transported from  user to kernal and back again. It is *not* a handle,
it is a cookie.

Handles for the kernel/user boundary should come from xarrays that
have nice lookup properties - not from user provided 64 bit values
that have to be stored in red black trees..

Jason