Re: [PATCH v8 2/3] vfio/ism: Implement vfio_pci driver for ISM devices

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

On 3/25/2026 6:31 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: Alexandra Winter<wintera@linux.ibm.com>
> Reviewed-by: Niklas Schnelle<schnelle@linux.ibm.com>
> Signed-off-by: Julian Ruess<julianr@linux.ibm.com>
> ---

I think we have decided to not support any additional regions with this 
patch. Feel free to add

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