linux-kernel - Re: [RFC 07/20] iommu/iommufd: Add iommufd_[un]bind

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Date:   Thu, 7 Oct 2021 12:23:13 +1100
From:   David Gibson <david@...son.dropbear.id.au>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Liu Yi L <yi.l.liu@...el.com>, alex.williamson@...hat.com,
        hch@....de, jasowang@...hat.com, joro@...tes.org,
        jean-philippe@...aro.org, kevin.tian@...el.com, parav@...lanox.com,
        lkml@...ux.net, pbonzini@...hat.com, lushenming@...wei.com,
        eric.auger@...hat.com, corbet@....net, ashok.raj@...el.com,
        yi.l.liu@...ux.intel.com, jun.j.tian@...el.com, hao.wu@...el.com,
        dave.jiang@...el.com, jacob.jun.pan@...ux.intel.com,
        kwankhede@...dia.com, robin.murphy@....com, kvm@...r.kernel.org,
        iommu@...ts.linux-foundation.org, dwmw2@...radead.org,
        linux-kernel@...r.kernel.org, baolu.lu@...ux.intel.com,
        nicolinc@...dia.com
Subject: Re: [RFC 07/20] iommu/iommufd: Add iommufd_[un]bind_device()

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.

> RDMA has many similar examples.
> 
> 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.

-- 
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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