On 1/19/21 9:02 PM, Matthew Rosato wrote:

Some s390 PCI devices (e.g. ISM) perform I/O operations that have very
specific requirements in terms of alignment as well as the patterns in
which the data is read/written. Allowing these to proceed through the
typical vfio_pci_bar_rw path will cause them to be broken in up in such a
way that these requirements can't be guaranteed. In addition, ISM devices
do not support the MIO codepaths that might be triggered on vfio I/O coming
from userspace; we must be able to ensure that these devices use the
non-MIO instructions.  To facilitate this, provide a new vfio region by
which non-MIO instructions can be passed directly to the host kernel s390
PCI layer, to be reliably issued as non-MIO instructions.

This patch introduces the new vfio VFIO_REGION_SUBTYPE_IBM_ZPCI_IO region
and implements the ability to pass PCISTB and PCILG instructions over it,
as these are what is required for ISM devices.

Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
---
 drivers/vfio/pci/vfio_pci.c         |   8 ++
 drivers/vfio/pci/vfio_pci_private.h |   6 ++
 drivers/vfio/pci/vfio_pci_zdev.c    | 158 ++++++++++++++++++++++++++++++++++++
 include/uapi/linux/vfio.h           |   4 +
 include/uapi/linux/vfio_zdev.h      |  33 ++++++++
 5 files changed, 209 insertions(+)

Related to the discussion on the QEMU side, if we have a check
to make sure this is only used for ISM, then this patch should
make that clear in its wording and also in the paths
(drivers/vfio/pci/vfio_pci_ism.c instead of vfio_pci_zdev.c.)
This also has precedent with the region for IGD in
drivers/vfio/pci/vfio_pci_igd.c.