Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

David Gibson
2021-04-27T05:15:24Z

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On Fri, Apr 23, 2021 at 07:28:03PM -0300, Jason Gunthorpe wrote:
> On Fri, Apr 23, 2021 at 10:38:51AM -0600, Alex Williamson wrote:
> > On Thu, 22 Apr 2021 20:39:50 -0300
>
> > > /dev/ioasid should understand the group concept somehow, otherwise it
> > > is incomplete and maybe even security broken.
> > >
> > > So, how do I add groups to, say, VDPA in a way that makes sense? The
> > > only answer I come to is broadly what I outlined here - make
> > > /dev/ioasid do all the group operations, and do them when we enjoin
> > > the VDPA device to the ioasid.
> > >
> > > Once I have solved all the groups problems with the non-VFIO users,
> > > then where does that leave VFIO? Why does VFIO need a group FD if
> > > everyone else doesn't?
> >
> > This assumes there's a solution for vDPA that doesn't just ignore the
> > problem and hope for the best. I can't speak to a vDPA solution.
>
> I don't think we can just ignore the question and succeed with
> /dev/ioasid.
>
> Guess it should get answered as best it can for ioasid "in general"
> then we can decide if it makes sense for VFIO to use the group FD or
> not when working in ioasid mode.
>
> Maybe a better idea will come up
>
> > an implicit restriction. You've listed a step in the description about
> > a "list of devices in the group", but nothing in the pseudo code
> > reflects that step.
>
> I gave it below with the readdir() - it isn't in the pseudo code
> because the applications I looked through didn't use it, and wouldn't
> benefit from it. I tried to show what things were doing today.

And chance are they will break cryptically if you give them a device
in a multi-device group. That's not something we want to encourage.

>
> > I expect it would be a subtly missed by any userspace driver
> > developer unless they happen to work on a system where the grouping
> > is not ideal.
>
> I'm still unclear - what are be the consequence if the application
> designer misses the group detail?
>
> 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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