IBM Z ISM (Internal Shared Memory) VFIO passthrough
ISM (Internal Shared Memory) is a virtual PCI device on IBM Z and LinuxONE mainframes that provides shared-memory communication between LPARs and KVM guests, used by the SMC-D protocol to accelerate TCP socket traffic. This VFIO variant exposes those ISM adapters to virtual machines so guests can use SMC-D directly on current-generation systems such as the IBM z17.
recommendation
It should stay because this is active, recent IBM work (a v6 patch series posted in early 2026) that lets KVM guests on IBM Z and LinuxONE get pass-through access to ISM virtual PCI adapters, which underpin the SMC-D high-speed inter-LPAR networking feature on currently sold z17 hardware. Deployment is low in absolute terms but real in mainframe shops, and there is no alternative path for the same mediated ISM use case.
repository signals
sources
- patchew.org
March 2026 patch series introduces a vfio_pci driver for ISM devices, indicating active upstream development rather than removal or abandonment.
- ibm.com
IBM documents ISM as an internal shared memory virtual PCI device used for SMC-D on Linux on IBM Z/LinuxONE.
- ibm.com
Current IBM z17 HMC documentation describes SMC-D partition links built on ISM adapters, showing the feature exists on current-generation systems.
- cateee.net
LKDDb lists CONFIG_ISM support across current kernel series and identifies IBM PCI device 1014:04ed as the ISM virtual PCI device.
codex reasoning notes (technical)
Local shell inspection found no merged drivers/vfio/pci/ism directory in this snapshot, but did find the established base ISM support in drivers/s390/net/ism_drv.c, so this dossier appears to target a pending/new VFIO variant rather than stale code. Upstream activity evidence came from web search result turn0search3 (Patchew patch series for vfio_pci ISM). Deployment and hardware-status evidence came from web search results turn1search0 and turn1search10 plus IBM's current Linux on IBM Z/z17 docs; LKDDb data came from web search result turn0search2. Conclusion: niche s390/LinuxONE virtualization support, still relevant on currently sold IBM Z/LinuxONE systems, with low-volume but real enterprise deployment and no obvious replacement for the same mediated/pass-through ISM use case.