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

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Date:   Mon, 11 Oct 2021 14:24:21 +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 Thu, Oct 07, 2021 at 08:35:03AM -0300, Jason Gunthorpe wrote:
> On Thu, Oct 07, 2021 at 12:23:13PM +1100, David Gibson wrote:
> > 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.
> 
> I'm not sure how that matters, the kernel has many handles - and we
> get to make more of them.. Look at xarray/idr users in the kernel, many of
> those are making userspace handles.

Again, I'm commenting *just* on the fact that the current draft
introduce *two* handles for the same object.  I have no objection to
either of the handles in isoation.

> > 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.
> 
> I don't think the user_data 64 bit value should be unique, it is just
> transported from  user to kernal and back again. It is *not* a handle,
> it is a cookie.
> 
> Handles for the kernel/user boundary should come from xarrays that
> have nice lookup properties - not from user provided 64 bit values
> that have to be stored in red black trees..

Yes, I think that would make more 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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