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

Niklas Schnelle
2026-03-26T13:04:23Z

On Wed, 2026-03-25 at 14:31 +0100, 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>
> ---
> 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 | 408 ++++++++++++++++++++++++++++++++++++++++++
> 5 files changed, 425 insertions(+)
>
--- snip ---
> +
> +static int ism_vfio_pci_ioctl_get_region_info(struct vfio_device *core_vdev,
> + struct vfio_region_info *info,
> + struct vfio_info_cap *caps)
> +{
> + struct vfio_pci_core_device *vdev =
> + container_of(core_vdev, struct vfio_pci_core_device, vdev);
> + struct pci_dev *pdev = vdev->pdev;
> +
> + switch (info->index) {
> + case VFIO_PCI_CONFIG_REGION_INDEX:
> + info->offset = ISM_VFIO_PCI_INDEX_TO_OFFSET(info->index);
> + info->size = pdev->cfg_size;
> + info->flags = VFIO_REGION_INFO_FLAG_READ |
> + VFIO_REGION_INFO_FLAG_WRITE;
> + break;
> + case VFIO_PCI_BAR0_REGION_INDEX ... VFIO_PCI_BAR5_REGION_INDEX:
> + info->offset = ISM_VFIO_PCI_INDEX_TO_OFFSET(info->index);
> + info->size = pci_resource_len(pdev, info->index);
> + if (!info->size) {
> + info->flags = 0;
> + break;
> + }
> + info->flags = VFIO_REGION_INFO_FLAG_READ |
> + VFIO_REGION_INFO_FLAG_WRITE;
> + break;
> + default:
> + info->offset = 0;
> + info->size = 0;
> + info->flags = 0;
> + return -EINVAL;

Thinking more about this, the above default handling actually breaks
additional regions such as the one added by Farhan for the error
events. I think this needs to instead call the generic
vfio_pci_ioctl_get_region_info() for other regions.

> + }
> + return 0;
> +}
--- snip ---