Re: [RFC 0/4] vfio-pci/zdev: Fixing s390 vfio-pci ISM support

[Cornelia Huck <cohuck@redhat.com>]

On Wed,  9 Dec 2020 15:27:46 -0500
Matthew Rosato <mjrosato@linux.ibm.com> wrote:

> Today, ISM devices are completely disallowed for vfio-pci passthrough as
> QEMU will reject the device due to an (inappropriate) MSI-X check.
> However, in an effort to enable ISM device passthrough, I realized that the
> manner in which ISM performs block write operations is highly incompatible
> with the way that QEMU s390 PCI instruction interception and
> vfio_pci_bar_rw break up I/O operations into 8B and 4B operations -- ISM
> devices have particular requirements in regards to the alignment, size and
> order of writes performed.  Furthermore, they require that legacy/non-MIO
> s390 PCI instructions are used, which is also not guaranteed when the I/O
> is passed through the typical userspace channels.

The part about the non-MIO instructions confuses me. How can MIO
instructions be generated with the current code, and why does changing
the write pattern help?

> 
> As a result, this patchset proposes a new VFIO region to allow a guest to
> pass certain PCI instruction intercepts directly to the s390 host kernel
> PCI layer for exeuction, pinning the guest buffer in memory briefly in
> order to execute the requested PCI instruction.
> 
> Matthew Rosato (4):
>   s390/pci: track alignment/length strictness for zpci_dev
>   vfio-pci/zdev: Pass the relaxed alignment flag
>   s390/pci: Get hardware-reported max store block length
>   vfio-pci/zdev: Introduce the zPCI I/O vfio region
> 
>  arch/s390/include/asm/pci.h         |   4 +-
>  arch/s390/include/asm/pci_clp.h     |   7 +-
>  arch/s390/pci/pci_clp.c             |   2 +
>  drivers/vfio/pci/vfio_pci.c         |   8 ++
>  drivers/vfio/pci/vfio_pci_private.h |   6 ++
>  drivers/vfio/pci/vfio_pci_zdev.c    | 160 ++++++++++++++++++++++++++++++++++++
>  include/uapi/linux/vfio.h           |   4 +
>  include/uapi/linux/vfio_zdev.h      |  33 ++++++++
>  8 files changed, 221 insertions(+), 3 deletions(-)
>