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Date:   Thu, 7 Oct 2021 12:23:13 +1100
From:   David Gibson <david@gibson.dropbear.id.au>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Liu Yi L <yi.l.liu@intel.com>, alex.williamson@redhat.com,
        hch@lst.de, jasowang@redhat.com, joro@8bytes.org,
        jean-philippe@linaro.org, kevin.tian@intel.com, parav@mellanox.com,
        lkml@metux.net, pbonzini@redhat.com, lushenming@huawei.com,
        eric.auger@redhat.com, corbet@lwn.net, ashok.raj@intel.com,
        yi.l.liu@linux.intel.com, jun.j.tian@intel.com, hao.wu@intel.com,
        dave.jiang@intel.com, jacob.jun.pan@linux.intel.com,
        kwankhede@nvidia.com, robin.murphy@arm.com, kvm@vger.kernel.org,
        iommu@lists.linux-foundation.org, dwmw2@infradead.org,
        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()
Message-ID: <YV5MAdzR6c2knowf@yekko>
References: <20210919063848.1476776-1-yi.l.liu@intel.com>
 <20210919063848.1476776-8-yi.l.liu@intel.com>
 <YVP44v4FVYJBSEEF@yekko>
 <20210929122457.GP964074@nvidia.com>
 <YVUqpff7DUtTLYKx@yekko>
 <20211001124322.GN964074@nvidia.com>
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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:
> > >=20
> > > > > +struct iommufd_device {
> > > > > +	unsigned int id;
> > > > > +	struct iommufd_ctx *ictx;
> > > > > +	struct device *dev; /* always be the physical device */
> > > > > +	u64 dev_cookie;
> > > >=20
> > > > 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?
> > >=20
> > > ID is the kernel value in the xarray - xarray is much more efficient &
> > > safe with small kernel controlled values.
> > >=20
> > > 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.
> > >=20
> > > It is a pretty normal pattern
> >=20
> > Hm, ok.  Could you point me at an example?
>=20
> 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.

> RDMA has many similar examples.
>=20
> More or less anytime you want to allow the kernel to async retun some
> information providing a 64 bit user_data lets userspace have an easier
> time to deal with it.

I absolutely see the need for user_data.  What I'm questioning is
having two different, user-visible unique handles, neither of which is
an fd.


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.

--=20
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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