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Date:   Wed, 29 Sep 2021 16:18:34 +1000
From:   "david@gibson.dropbear.id.au" <david@gibson.dropbear.id.au>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     "Tian, Kevin" <kevin.tian@intel.com>,
        "Liu, Yi L" <yi.l.liu@intel.com>,
        "alex.williamson@redhat.com" <alex.williamson@redhat.com>,
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        "jean-philippe@linaro.org" <jean-philippe@linaro.org>,
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        "pbonzini@redhat.com" <pbonzini@redhat.com>,
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        "eric.auger@redhat.com" <eric.auger@redhat.com>,
        "corbet@lwn.net" <corbet@lwn.net>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "yi.l.liu@linux.intel.com" <yi.l.liu@linux.intel.com>,
        "Tian, Jun J" <jun.j.tian@intel.com>, "Wu, Hao" <hao.wu@intel.com>,
        "Jiang, Dave" <dave.jiang@intel.com>,
        "jacob.jun.pan@linux.intel.com" <jacob.jun.pan@linux.intel.com>,
        "kwankhede@nvidia.com" <kwankhede@nvidia.com>,
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        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        "dwmw2@infradead.org" <dwmw2@infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "baolu.lu@linux.intel.com" <baolu.lu@linux.intel.com>,
        "nicolinc@nvidia.com" <nicolinc@nvidia.com>
Subject: Re: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO
Message-ID: <YVQFOun0Ae3/V2Y4@yekko>
References: <20210919063848.1476776-1-yi.l.liu@intel.com>
 <20210919063848.1476776-11-yi.l.liu@intel.com>
 <20210921174049.GV327412@nvidia.com>
 <BN9PR11MB5433D26EFA94F59756AF91838CA29@BN9PR11MB5433.namprd11.prod.outlook.com>
 <20210922124150.GK327412@nvidia.com>
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On Wed, Sep 22, 2021 at 09:41:50AM -0300, Jason Gunthorpe wrote:
> On Wed, Sep 22, 2021 at 03:30:09AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@nvidia.com>
> > > Sent: Wednesday, September 22, 2021 1:41 AM
> > >=20
> > > 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 inte=
rface
> > > > to query the underlying iommu capability and format info for this d=
evice.
> > > > Based on this information the user then creates I/O address space i=
n a
> > > > compatible format with the to-be-attached devices.
> > > >
> > > > Device cookie which is registered at binding time is used to mark t=
he
> > > > device which is being queried here.
> > > >
> > > > Signed-off-by: Liu Yi L <yi.l.liu@intel.com>
> > > >  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)
> > > > +{
> > >=20
> > > We have an xarray ID for the device, why are we allowing userspace to
> > > use the dev_cookie as input?
> > >=20
> > > 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.
> > >=20
> >=20
> > A little background.
> >=20
> > 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.
> >=20
> > Now you instead recommended a mixed option. We can follow it for
> > sure if nobody objects.
>=20
> Either or for the return is fine, I'd return both just because it is
> more flexable
>=20
> But the cookie should never be an input from userspace, and the kernel
> should never search for it. Locating the kernel object is what the ID
> and xarray is for.

Why do we need two IDs at all?  Can't we just use the cookie as the
sole ID?

--=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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