Re: [PATCH 4/4] vfio-pci/zdev: Introduce the zPCI I/O vfio region

From: Niklas Schnelle
Date: Thu Jan 21 2021 - 05:03:57 EST

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@xxxxxxxxxxxxx>
> ---
> 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