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Date:   Wed, 29 Sep 2021 09:24:57 -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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Subject: Re: [RFC 07/20] iommu/iommufd: Add iommufd_[un]bind_device()
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References: <20210919063848.1476776-1-yi.l.liu@intel.com>
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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

Jason