linux-kernel - Re: [RFC 13/20] iommu: Extend iommu_at[de]tach

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Message-ID: <YXauO+YSR7ivz1QW@yekko>
Date:   Tue, 26 Oct 2021 00:16:43 +1100
From:   David Gibson <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>,
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        "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>,
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        "nicolinc@...dia.com" <nicolinc@...dia.com>
Subject: Re: [RFC 13/20] iommu: Extend iommu_at[de]tach_device() for multiple
 devices group

On Mon, Oct 25, 2021 at 09:14:10AM -0300, Jason Gunthorpe wrote:
> On Mon, Oct 25, 2021 at 04:14:56PM +1100, David Gibson wrote:
> > On Mon, Oct 18, 2021 at 01:32:38PM -0300, Jason Gunthorpe wrote:
> > > On Mon, Oct 18, 2021 at 02:57:12PM +1100, David Gibson wrote:
> > > 
> > > > The first user might read this.  Subsequent users are likely to just
> > > > copy paste examples from earlier things without fully understanding
> > > > them.  In general documenting restrictions somewhere is never as
> > > > effective as making those restrictions part of the interface signature
> > > > itself.
> > > 
> > > I'd think this argument would hold more water if you could point to
> > > someplace in existing userspace that cares about the VFIO grouping.
> > 
> > My whole point here is that the proposed semantics mean that we have
> > weird side effects even if the app doesn't think it cares about
> > groups.
> > 
> > e.g. App's input is a bunch of PCI addresses for NICs.  It attaches
> > each one to a separate IOAS and bridges packets between them all.  As
> > far as the app is concerned, it doesn't care about groups, as you say.
> > 
> > Except that it breaks if any two of the devices are in the same group.
> > Worse, it has a completely horrible failure mode: no syscall returns
> 
> Huh? If an app requests an IOAS attach that is not possible then the
> attachment IOCTL will fail.
> 
> The kernel must track groups and know that group A is on IOAS A and
> any further attach of a group A device must specify IOAS A or receive
> a failure.

Ok, I misunderstood the semantics that were suggested.

So, IIUC what you're suggested is that if group X is attached to IOAS
1, then attaching the group to IOAS 1 again should succeed (as a
no-op), but attaching to any other IOAS should fail?

That's certainly an improvement, but there's still some questions.

If you attach devices A and B (both in group X) to IOAS 1, then detach
device A, what happens?  Do you detach both devices?  Or do you have a
counter so you have to detach as many time as you attached?

> The kernel should never blindly acknowledge a failed attachment.
> 
> Jason
> 

-- 
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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