Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

On Tue, May 02, 2023 at 03:28:40PM +0200, David Hildenbrand wrote:

On 02.05.23 15:10, Jason Gunthorpe wrote:

quoted

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

quoted

quoted

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