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

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Date:   Tue, 27 Apr 2021 15:15:05 +1000
From:   David Gibson <david@...son.dropbear.id.au>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Auger Eric <eric.auger@...hat.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        David Woodhouse <dwmw2@...radead.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Jonathan Corbet <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Alexey Kardashevskiy <aik@...abs.ru>
Subject: Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and
 allocation APIs

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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