Nested KVM on POWER9 means running a guest hypervisor inside a virtual machine so that it can host further virtual machines. A Class Central listing for an advanced, 46-minute linux.conf.au talk maps the key questions—KVM-PR versus KVM-HV, guest entry and exit, address translation, memory management, migration, performance, and nesting depth—but does not establish current compatibility, implementation status, or operating limits. This is an architectural guide to the talk’s scope, not a deployment recipe.
What nested KVM means
In a nested virtualization arrangement, the physical system hosts a first-level virtual machine (L1), and a hypervisor running within L1 can create another virtual machine (L2). The physical host is often called L0. The central challenge is coordinating the virtualization responsibilities across those layers: the L1 hypervisor must behave as a hypervisor for L2 while itself running as a guest.
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The linux.conf.au talk listing frames nested virtualization as running virtual machines inside virtual machines. Its syllabus names the design and operational questions that follow, but its outline is not a technical specification of how a particular POWER9 system handles them.
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A nested guest has to enter and leave execution through more than one virtualization layer. The talk outline identifies nested KVM-HV entry and exit as a core topic. That makes the transition path an important point for engineers to investigate: how control is handed between levels, and what work the implementation performs, affect both correctness and overhead. The listing does not describe the implementation details, so it cannot support a specific account of POWER9’s entry or exit mechanism.
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Memory management is another central topic. The outline specifically names nested guest address translation, partition-scoped PTE generation, and process- and partition-scoped invalidations. These are areas where a nested implementation must manage address-space state and keep translation information coherent across virtualization levels. The listing identifies the subjects, but gives no algorithms, limits, or configuration instructions for them.
KVM-PR and KVM-HV: what the outline establishes
The talk treats KVM-PR and KVM-HV as approaches to compare. It raises hardware support, guest entry and exit, address translation, invalidation behavior, migration, performance, and practical configuration as comparison axes. The Class Central listing does not report comparative outcomes, so it does not establish that one approach is faster, more capable, or preferable for a given workload.
| Comparison question | What the talk listing establishes | What it does not establish |
|---|---|---|
| Hardware support | KVM-PR and KVM-HV are named as approaches for discussion. | A POWER9 model, firmware, or configuration compatibility matrix. |
| Guest entry and exit | Nested KVM-HV entry and exit are listed as topics. | A detailed control-flow description or version-specific behavior. |
| Address translation and memory management | Nested guest address translation and memory-management topics are listed. | Translation algorithms, limits, or measured overhead. |
| PTE generation and invalidations | Partition-scoped PTE generation and process- and partition-scoped invalidations are named. | Implementation details or comparative results. |
| Migration | Migration between virtualization levels is raised. | Supported migration paths, prerequisites, or guarantees. |
| Performance and practical use | Performance and practical configuration are included in the outline. | Benchmarks, deployment steps, or a recommended setup. |
How nested KVM on POWER9 differs from KVM inside a PowerVM LPAR
IBM’s 2024 PowerVM article describes a related but distinct arrangement: a KVM guest (L2) runs inside a Linux LPAR (L1), while PowerVM (L0) assigns CPU, memory, and I/O resources to that LPAR. IBM associates the described KVM-in-LPAR feature with PowerVM firmware FW1060.10.
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This is useful context for understanding the layer labels, but it should not be treated as proof of a particular nested KVM-on-POWER9 configuration. The PowerVM article describes a PowerVM resource layer; the linux.conf.au talk listing concerns nested KVM on POWER9. They are not interchangeable support claims.
How deep can you nest?
The talk outline itself poses the question of nesting depth, but the listing supplies no answer. It gives no maximum number of levels, no tested configuration, and no distinction between a theoretical capability and a supported operational limit. A depth claim therefore needs evidence for the precise hardware, kernel, QEMU, and firmware combination being discussed; the listing alone cannot provide it.
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The listing is a useful map for evaluating an implementation, not enough to qualify a production configuration. Before relying on nested KVM, confirm the details against current primary documentation and implementation sources for the exact system under consideration.
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- Identify the POWER9 system model and firmware, along with the host and guest kernel and QEMU versions.
- Verify that the intended host and guest virtualization modes are supported in that specific combination.
- Check how nested guest entry, address translation, PTE generation, and invalidations are implemented and bounded.
- Establish which migration paths are supported across levels and what state or device restrictions apply.
- Look for performance measurements that match the intended workload and configuration rather than inferring performance from the talk outline.
- Test the required nesting depth and device behavior in a non-production environment before depending on them.
These checks matter because the available talk listing does not state a current code status, compatibility matrix, deployment procedure, or operational limits.
Hardware and further learning
An OpenPOWER Foundation event post names a Raptor Computing Systems Blackbird POWER9 motherboard as hardware the presenter planned to bring for show and tell. That is a historical example of relevant POWER9 hardware, not confirmation of present-day availability or compatibility with a nested KVM setup. A lab system may be useful for hands-on evaluation, but the exact platform must be checked against the software and firmware requirements.
The linux.conf.au presentation is a relevant starting point for the architecture topics it covers. Treat its outline as a guide to questions—rather than as evidence for current support, performance figures, migration guarantees, or a maximum nesting depth.
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