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The presentation, “Hard Partitioning for Linux: The Jailhouse Hypervisor,” was given by Jan Kiszka of Siemens Corporate Technology in August 2015. The co-located KVM Forum 2015 took place in Seattle on August 19–21. KVM Forum 2015 archive
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What is Jailhouse?
Jailhouse is a Linux-managed hypervisor designed to isolate workloads on multicore hardware. The already-booted Linux system remains in the root cell. Jailhouse can then allocate selected CPUs and devices to non-root cells, which may run an RTOS, bare-metal code, or another Linux instance. The hypervisor enforces the separation; Linux handles boot, cell loading and starting, control, and monitoring. The project is GPLv2-licensed and favors a minimal feature set over a broad virtualization layer. Jailhouse project
What makes Jailhouse different?
The central idea is hard partitioning, not general-purpose virtual-machine scheduling. Rather than virtualizing access to every resource or time-sharing CPUs, Jailhouse assigns resources to cells on a 1:1 basis and relies on access controls for isolation. It partitions a system after Linux has booted, and it does not try to conceal the hypervisor from Linux.
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- Fixed assignments: CPUs and devices belong to designated cells rather than being dynamically scheduled across them.
- Linux remains in charge: The root cell supplies the management environment; Jailhouse itself is not intended to replace the host OS.
- Limited complexity: The design emphasizes simplicity and isolation rather than extensive guest features.
- Real-time and safety goals: The stated aim was bare-metal-like performance and latency for selected workloads, not a universal guarantee for every board or configuration.
How Linux and non-root cells fit together
In the standard arrangement, Linux runs in the root cell and manages the system. A non-root cell can host code that needs a dedicated set of cores or devices. This model allows Linux services and ordinary system functions to coexist with a more tightly isolated workload, while keeping management and boot operations in Linux.
The slides described two management models. In the open model, Linux alone controls management decisions. In the safety model, selected cells can vote on management decisions, with Linux still in control. The presentation framed that voting arrangement as a building block for safe operation, not as proof of certification or a complete safety case. Siemens presentation slides
Linux as a non-root cell?
Kiszka also addressed running Linux in a non-root cell, including multiple Linux instances on one machine. At the August 2015 talk, the reported x86 status included working SMP, MSI/MSI-X PCI assignment, and inter-cell shared memory. Legacy INTx interrupts were not yet supported.
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ARM had additional shared-resource complications. The slides cited clock-gate control on Banana Pi and said there was no publicly available reference setup at that time. These are dated implementation details from the 2015 presentation, not a statement of current board support.
Can you also run Linux partitions?
Yes. The presentation showed Linux running in a non-root cell, alongside the root Linux instance. The design’s static allocation model matters here: each Linux instance needs its own assigned resources, and the talk’s reported x86 and ARM caveats show that hardware and device behavior affected feasibility. This was not presented as an unrestricted ability to launch arbitrary Linux guests on any machine.
How did cells communicate?
The 2015 slides described ivshmem: a read/write shared-memory region available to two cells, accompanied by MSI signaling. The design sought to minimize copying, hypervisor work, and dynamic page remapping. The presentation explicitly noted that no messaging layer had yet been built on top, so applications would need to define or provide higher-level communication behavior themselves. Siemens presentation slides
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Why not use Xen PV interfaces?
Jailhouse’s answer follows from its design priorities. Its inter-cell communication approach was intended to keep the hypervisor small and avoid extra copying or dynamic remapping. A richer paravirtualized interface would add a layer of guest/hypervisor interaction; the slides instead describe shared memory plus interrupt signaling, without a messaging layer. This is a design rationale, not evidence that Xen interfaces are unsuitable for other workloads.
How was Jailhouse configured?
The workflow used descriptions of the system, root cell, and additional cells. The deck characterized the format as precise and flexible, but not yet convenient.
- Generate a starting system configuration with
jailhouse config create my-system.c. - Review and post-process the generated C configuration manually to account for the system and intended partitions.
- Compile
my-system.cintomy-system.cell; cell configurations were derived from the system configuration.
The need to inspect and refine low-level resource descriptions made configuration exacting. Kiszka also cautioned in the Jailhouse 0.5 announcement that real-hardware use could require fine-tuning and deeper understanding. Jailhouse 0.5 announcement, 11 May 2015
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What did the August 2015 implementation support?
The figures below are Siemens Corporate Technology’s status snapshot in the August 2015 presentation. They should not be read as current specifications or as guarantees for other hardware.
| Area | Reported status in August 2015 |
|---|---|
| Intel code size | Approximately 8.5K lines of code, as reported by Siemens Corporate Technology in 2015. |
| Latency | Maximum timer IRQ latency below 2.5 µs on a Xeon D-1540, as reported by Siemens Corporate Technology in 2015. |
| ARMv7 code size | Approximately 6.5K lines for the TK1 implementation, as reported by Siemens Corporate Technology in 2015. |
| Intel hardware requirements | VT-x/VT-d or AMD-V was required in the presentation’s support snapshot. |
| ARMv7 platforms | FastModel, Banana Pi, and NVIDIA Jetson TK1 were listed in the presentation. |
| ARMv8 | Patches were progressing but were not yet working in the presentation’s status snapshot. |
The contemporaneous Jailhouse 0.5 release added AMD64; ARMv7 support for Banana Pi, NVIDIA Jetson TK1, and Versatile Express; foundations for ivshmem inter-cell communication; improved x86 isolation; and support for larger x86 machines. Those release notes also warned that real hardware could require tuning. Jailhouse 0.5 announcement, 11 May 2015
What did the demonstrations show?
The slides document demonstrations of Jailhouse running inside QEMU/KVM and of Jailhouse booting Linux. They establish what was shown during the presentation; they are not independent benchmarks or reproductions.
Who was Jailhouse for?
The 2015 presentation positioned Jailhouse for developers who needed selected real-time, safety-oriented, or security-sensitive work isolated on multicore hardware while retaining Linux as the management environment. Its fixed allocations and low-overhead design came with trade-offs: hardware-specific configuration, limited convenience, and a narrower feature set than a general-purpose hypervisor. The reported performance, architecture support, and integration status belong specifically to the August 2015 snapshot.
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