[PATCH v2 0/3] vfio/pci: s390: Fix issues preventing VFIO_PCI_MMAP=y for s390 and enable it

Niklas Schnelle
2024-05-23T11:10:33Z

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 and signed
b4/vfio_pci_mmap branch on my git.kernel.org site below:
https: //git.kernel.org/pub/scm/linux/kernel/git/niks/linux.git/

Thanks,
Niklas

Link: https://lore.kernel.org/all/c5ba134a1d4f4465b5956027e6a4ea6f6beff969.camel@linux.ibm.com/
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
---
Changes in v2:
- Changed last patch to remove VFIO_PCI_MMAP instead of just enabling it
for s390 as it is unconditionally true with s390 supporting PCI resource mmap() (Jason)
- Collected R-bs from Jason
- Link to v1: https://lore.kernel.org/r/20240521-vfio_pci_mmap-v1-0-2f6315e0054e@linux.ibm.com

---
Niklas Schnelle (3):
s390/pci: Fix s390_mmio_read/write syscall page fault handling
vfio/pci: Tolerate oversized BARs by disallowing mmap
vfio/pci: Enable PCI resource mmap() on s390 and remove VFIO_PCI_MMAP

arch/s390/pci/pci_mmio.c | 18 +++++++++++++-----
drivers/vfio/pci/Kconfig | 4 ----
drivers/vfio/pci/vfio_pci_core.c | 11 ++++++-----
3 files changed, 19 insertions(+), 14 deletions(-)
---
base-commit: a38297e3fb012ddfa7ce0321a7e5a8daeb1872b6
change-id: 20240503-vfio_pci_mmap-1549e3d02ca7

Best regards,
--
Niklas Schnelle