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Date:   Tue, 2 May 2023 10:36:37 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Christian Borntraeger <borntraeger@linux.ibm.com>,
        Lorenzo Stoakes <lstoakes@gmail.com>, linux-mm@kvack.org,
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        Oleg Nesterov <oleg@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>, Jan Kara <jack@suse.cz>,
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        Pavel Begunkov <asml.silence@gmail.com>,
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        Dave Chinner <david@fromorbit.com>,
        Theodore Ts'o <tytso@mit.edu>, Peter Xu <peterx@redhat.com>,
        Matthew Rosato <mjrosato@linux.ibm.com>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing
 to file-backed mappings
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On Tue, May 02, 2023 at 03:28:40PM +0200, David Hildenbrand wrote:
> On 02.05.23 15:10, Jason Gunthorpe wrote:
> > On Tue, May 02, 2023 at 03:04:27PM +0200, Christian Borntraeger wrote:
> > \> > We can reintroduce a flag to permit exceptions if this is really broken, are you
> > > > able to test? I don't have an s390 sat around :)
> > > 
> > > Matt (Rosato on cc) probably can. In the end, it would mean having
> > >    <memoryBacking>
> > >      <source type="file"/>
> > >    </memoryBacking>
> > 
> > This s390 code is the least of the problems, after this series VFIO
> > won't startup at all with this configuration.
> 
> Good question if the domain would fail to start. I recall that IOMMUs for
> zPCI are special on s390x. [1]

Not upstream they aren't.

> Well, zPCI is special. I cannot immediately tell when we would trigger
> long-term pinning.

zPCI uses the standard IOMMU stuff, so it uses a normal VFIO container
and the normal pin_user_pages() path.

> [1] https://www.mail-archive.com/qemu-devel@nongnu.org/msg875728.html

AFIACT this is talking about the nested IOMMU translation stuff.
RPCIT is the hypercall to invalidate the nested IOMMU table. I expect s390 is
going to have another go at implementing this using iommufd.

The stuff in that series like KVM_S390_ZPCIOP_REG_IOAT didn't make it
upstream.

Jason