[PATCH v5 0/2] vfio/pci: s390: Fix issues preventing VFIO_PCI_MMAP=y for s390 and enable it

Niklas Schnelle
2025-02-12T15:29:42Z

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. Solve this
by keeping the vfio-pci mmap() blocking behavior around for this
specific device via a PCI quirk and new pdev->non_mappable_bars
flag.

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/

Thanks,
Niklas

Link: https://lore.kernel.org/all/c5ba134a1d4f4465b5956027e6a4ea6f6beff969.camel@linux.ibm.com/
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
---
Changes in v5:
- Instead of relying on the existing pdev->non_compliant_bars introduce
a new pdev->non_mappable_bars flag. This replaces the VFIO_PCI_MMAP
Kconfig option and makes it per-device. This is necessary to not break
upcoming vfio-pci use of ISM devices (Julian Ruess)
- Squash the removal of VFIO_PCI_MMAP into the second commit as this
is now where its only use goes away.
- Switch to using follow_pfnmap_start() in MMIO syscall page fault
handling to match upstream changes
- Dropped R-b's because the changes are significant
- Link to v4: https://lore.kernel.org/r/20240626-vfio_pci_mmap-v4-0-7f038870f022@linux.ibm.com

Changes in v4:
- Overhauled and split up patch 2 which caused errors on ppc due to
unexported __kernel_io_end. Replaced it with a minimal s390 PCI fixup
harness to set pdev->non_compliant_bars for ISM plus ignoring devices
with this flag in vfio-pci. Idea for using PCI quirks came from
Christoph Hellwig, thanks. Dropped R-bs for patch 2 accordingly.
- Rebased on v6.10-rc5 which includes the vfio-pci mmap fault handler
fix to the issue I stumbled over independently in v3
- Link to v3: https://lore.kernel.org/r/20240529-vfio_pci_mmap-v3-0-cd217d019218@linux.ibm.com

Changes in v3:
- Rebased on v6.10-rc1 requiring change to follow_pte() call
- Use current->mm for fixup_user_fault() as seems more common
- Collected new trailers
- Link to v2: https://lore.kernel.org/r/20240523-vfio_pci_mmap-v2-0-0dc6c139a4f1@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 (2):
s390/pci: Fix s390_mmio_read/write syscall page fault handling
PCI: s390: Support mmap() of BARs and replace VFIO_PCI_MMAP by a device flag

arch/s390/Kconfig | 4 +---
arch/s390/pci/Makefile | 2 +-
arch/s390/pci/pci_fixup.c | 23 +++++++++++++++++++++++
arch/s390/pci/pci_mmio.c | 18 +++++++++++++-----
drivers/s390/net/ism_drv.c | 1 -
drivers/vfio/pci/Kconfig | 4 ----
drivers/vfio/pci/vfio_pci_core.c | 2 +-
include/linux/pci.h | 1 +
include/linux/pci_ids.h | 1 +
9 files changed, 41 insertions(+), 15 deletions(-)
---
base-commit: a64dcfb451e254085a7daee5fe51bf22959d52d3
change-id: 20240503-vfio_pci_mmap-1549e3d02ca7

Best regards,
--
Niklas Schnelle