linux-kernel - Re: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET

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Message-ID: <YVQFOun0Ae3/V2Y4@yekko>
Date:   Wed, 29 Sep 2021 16:18:34 +1000
From:   "david@...son.dropbear.id.au" <david@...son.dropbear.id.au>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "hch@....de" <hch@....de>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "lkml@...ux.net" <lkml@...ux.net>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "lushenming@...wei.com" <lushenming@...wei.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "yi.l.liu@...ux.intel.com" <yi.l.liu@...ux.intel.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "nicolinc@...dia.com" <nicolinc@...dia.com>
Subject: Re: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO

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@...dia.com>
> > > 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@...el.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)
> > > > +{
> > > 
> > > 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.
> 
> Either or for the return is fine, I'd return both just because it is
> more flexable
> 
> 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?

-- 
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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