Re: [PATCH v3 0/3] vfio/pci: s390: Fix issues preventing VFIO_PCI_MMAP=y for s390 and enable it

[Niklas Schnelle]

On Mon, 2024-06-03 at 17:50 +0200, Christian Borntraeger wrote:
> Am 29.05.24 um 13:36 schrieb Niklas Schnelle:
> > With the introduction of memory I/O (MIO) instructions enbaled in commit
> > 71ba41c9b1d9 ("s390/pci: provide support for MIO instructions") s390
> > gained support for direct user-space access to mapped PCI resources.
> > Even without those however user-space can access mapped PCI resources
> > via the s390 specific MMIO syscalls. There is thus nothing fundamentally
> > preventing s390 from supporting VFIO_PCI_MMAP allowing user-space drivers
> > to access PCI resources without going through the pread() interface.
> > To actually enable VFIO_PCI_MMAP a few issues need fixing however.
> > 
> > Firstly the s390 MMIO syscalls do not cause a page fault when
> > follow_pte() fails due to the page not being present. This breaks
> > vfio-pci's mmap() handling which lazily maps on first access.
> > 
> > Secondly on s390 there is a virtual PCI device called ISM which has
> > a few oddities. For one it claims to have a 256 TiB PCI BAR (not a typo)
> > which leads to any attempt to mmap() it fail with the following message:
> > 
> >      vmap allocation for size 281474976714752 failed: use vmalloc=<size> to increase size
> > 
> > Even if one tried to map this BAR only partially the mapping would not
> > be usable on systems with MIO support enabled. So just block mapping
> > BARs which don't fit between IOREMAP_START and IOREMAP_END.
> > 
> > Note:
> > For your convenience the code is also available in the tagged
> > b4/vfio_pci_mmap branch on my git.kernel.org site below:
> > https: //git.kernel.org/pub/scm/linux/kernel/git/niks/linux.git/
> 
> 
> I guess its now mostly a question of who picks those patches? Alex?

That matches my understanding as well.

> 
> Any patch suitable for stable?

I'd almost say all but the last one may be candidates for stable. I
found it hard to pinpoint a specific commit they fix though, hence the
lack of Fixes tag. For the first one I'm actually not sure if e.g.
rdma-core users could also run into this problem when they get swapped
out as I'm not sure if the mapping is pinned there.