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Date:   Tue, 9 Mar 2021 12:47:43 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Alex Williamson <alex.williamson@redhat.com>
Cc:     cohuck@redhat.com, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, peterx@redhat.com
Subject: Re: [PATCH v1 07/14] vfio: Add a device notifier interface
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On Tue, Mar 09, 2021 at 08:45:13AM -0700, Alex Williamson wrote:

> > I'm having trouble guessing why we need to refcount the group to add a
> > notifier to the device's notifier chain? 
> > 
> > I suppose it actually has to do with the MMIO mapping? But I don't
> > know what the relation is between MMIO mappings in the IOMMU and the
> > container? This could deserve a comment?
> 
> Sure, I can add a comment.  We want to make sure the device remains
> within an IOMMU context so long as we have a DMA mapping to the device
> MMIO, which could potentially manipulate the device.  IOMMU context is
> managed a the group level.

I find refcounting is easier to understand if the refcount inc/dec is
near the thing that is actually using the object - so I'd suggest to
move this to the iommu code.

A comment sounds like a good idea since this is security sensitive

Jason