Re: [PATCH v3 2/3] vfio/ism: Implement vfio_pci driver for ISM devices

[Farhan Ali <alifm@linux.ibm.com>]

On 3/5/2026 4:20 AM, Julian Ruess wrote:
> Add a vfio_pci variant driver for the s390-specific Internal Shared
> Memory (ISM) devices used for inter-VM communication.
>
> This enables the development of vfio-pci-based user space drivers for
> ISM devices.
>
> On s390, kernel primitives such as ioread() and iowrite() are switched
> over from function handle based PCI load/stores instructions to PCI
> memory-I/O (MIO) loads/stores when these are available and not
> explicitly disabled. Since these instructions cannot be used with ISM
> devices, ensure that classic function handle-based PCI instructions are
> used instead.
>
> The driver is still required even when MIO instructions are disabled, as
> the ISM device relies on the PCI store block (PCISTB) instruction to
> perform write operations.
>
> Stores are not fragmented, therefore one ioctl corresponds to exactly
> one PCISTB instruction. User space must ensure to not write more than
> 4096 bytes at once to an ISM BAR which is the maximum payload of the
> PCISTB instruction.
>
> Reviewed-by: Niklas Schnelle<schnelle@linux.ibm.com>
> Signed-off-by: Julian Ruess<julianr@linux.ibm.com>
> ---
>   drivers/vfio/pci/Kconfig      |   2 +
>   drivers/vfio/pci/Makefile     |   2 +
>   drivers/vfio/pci/ism/Kconfig  |  10 ++
>   drivers/vfio/pci/ism/Makefile |   3 +
>   drivers/vfio/pci/ism/main.c   | 343 ++++++++++++++++++++++++++++++++++++++++++
>   5 files changed, 360 insertions(+)

Reviewed-by: Farhan Ali<alifm@linux.ibm.com>