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libvirt cannot set an arbitrary CPU cache capacity such as “8 MiB of L3” through its documented <cpu><cache> element. That element controls what cache information the guest sees: emulate supplies synthetic data, passthrough exposes host-reported data, and disable hides it. If you need to describe a virtual cache hierarchy, QEMU has separate smp-cache machine properties in applicable configurations, but support depends on the installed QEMU release, architecture, machine type, and CPU model.
First determine what “set CPU cache size” means
The error or unexpected result can represent several different goals. They are not interchangeable:
- Report the host cache: let the guest inspect cache information associated with the physical CPU.
- Present synthetic cache information: make the virtual CPU expose an emulated cache hierarchy.
- Hide cache information: prevent the guest from reporting cache levels.
- Allocate or partition real hardware cache: reserve a specified amount of host cache for a VM. The documented libvirt cache element does not establish this as a resource-control feature.
Only the first three are covered by libvirt’s documented cache element. The element has a mode and an optional level; it has no documented numeric size attribute. See the libvirt Domain XML reference.
What libvirt’s cache XML actually does
| Guest-facing goal | Configuration | What it means | Important limitation |
|---|---|---|---|
| Expose host cache information | mode='passthrough' |
Passes through cache data reported by the host CPU. | The result follows the host CPU and interacts with CPU-model and migration choices. |
| Provide synthetic cache information | mode='emulate' |
Supplies fake cache data to the guest. | It is not documented as a general control for choosing an arbitrary capacity in MiB. |
| Hide cache information | mode='disable' |
Suppresses cache reporting for the selected level, or for all levels when no level is supplied. | Hiding a report does not remove or resize the host’s physical cache. |
The optional level attribute selects the cache level described by the element. For example:
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<cpu>
<cache level='3' mode='emulate'/>
</cpu>Without level, the element describes all CPU cache levels at once. Elements that specify a level cannot be mixed with elements that omit it. If the cache element is absent, the hypervisor uses a sensible default, according to libvirt’s documentation.
Valid libvirt examples—and what they do not promise
Emulate cache information
<cpu>
<cache level='3' mode='emulate'/>
</cpu>This asks libvirt/QEMU to present emulated information for level 3. It does not mean that the VM receives a physically reserved 3 MiB or any other specified amount.
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Pass through host-reported cache data
<cpu mode='host-passthrough' migratable='off'>
<cache mode='passthrough'/>
</cpu>This combination is appropriate only when exposing the host CPU is acceptable. Host passthrough can reduce live-migration portability because another host may not provide a compatible CPU model or cache description. QEMU’s CPU-model guidance recommends a compatible model across hosts when migration is required, and host passthrough when migration is not needed; verify the guidance for your installed release at QEMU CPU model configuration.
Disable cache reporting
<cpu>
<cache mode='disable'/>
</cpu>With no level, this suppresses reporting for all cache levels covered by the element. It does not change the underlying hardware resources used by the VM.
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Why a numeric cache-size attribute fails
XML such as <cache size='8192'/> is not a documented libvirt interface for assigning an L3 capacity. A failure may therefore be an XML schema rejection before QEMU starts. A different failure may occur later when QEMU rejects a property that the selected machine type or CPU model does not support. A guest can also boot successfully while reporting cache information different from what you expected.
Separate those cases by reading the complete error text:
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- Validation error: the XML structure, attribute, nesting, or level combination is not accepted by libvirt.
- QEMU startup error: libvirt accepted the XML, but the generated QEMU command line or selected machine/CPU configuration is unsupported.
- Guest-observation problem: the VM starts, but its operating system reports a different cache topology or size.
QEMU’s separate cache-topology path
Current QEMU documentation describes smp-cache machine properties for supported configurations. These properties can describe cache types and hierarchy levels, including L1 data, L1 instruction, L2 unified, and L3 unified caches. This is a QEMU machine-option path, not a universal libvirt XML attribute for setting arbitrary cache capacity.
Check the documentation for the exact QEMU release installed on the host rather than assuming that the current master manual applies. Confirm the architecture, machine type, and CPU model as well. The relevant references are QEMU User Documentation and QEMU CPU model configuration. A property shown there may still be unavailable for a particular combination.
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A diagnostic procedure for a rejected or ineffective configuration
- Write down the intended result. Decide whether you want host data, synthetic topology, hidden data, or actual hardware-cache allocation. Do not troubleshoot these as one feature.
- Capture the full domain XML. Inspect both the running definition and the persistent definition, then check the
<cpu>nesting, CPU mode, cache mode, and everylevelattribute. For example, usevirsh dumpxml VMNAMEfor the active definition andvirsh dumpxml --inactive VMNAMEfor the persistent definition. - Check versions and platform details. Record
virsh version, the QEMU version, guest architecture, machine type, host CPU vendor/model, and selected CPU mode. Cache and CPU options are version- and configuration-dependent. - Inspect capabilities. Run
virsh domcapabilitiesfor the relevant emulator, architecture, and machine configuration. The resulting capabilities XML shows CPU modes and model information exposed by that libvirt/QEMU/host combination; see the libvirt domain capabilities reference. - Correct libvirt-level mistakes. Remove undocumented numeric size attributes, use one documented mode, and avoid mixing cache elements with and without
level. If no special guest presentation is required, removing the cache element lets the hypervisor choose its default. - Validate QEMU-specific topology support. If synthetic hierarchy is the goal, compare the exact installed QEMU manual with the selected architecture, machine type, and CPU model before adding
smp-cacheproperties. - Test migration separately. If live migration matters, compare CPU-model compatibility on every destination host. A host-passthrough setup that works locally can be unsuitable across heterogeneous hosts.
Choosing the right approach
| If you need… | Use or investigate… | Trade-off |
|---|---|---|
| The guest to see the physical host’s cache description | libvirt mode='passthrough' |
Couples the guest view to host CPU information and may complicate migration. |
| A guest-visible synthetic cache description | libvirt mode='emulate'; for a detailed hierarchy, applicable QEMU smp-cache properties |
Emulation/topology support is configuration- and version-dependent, and does not by itself reserve host cache. |
| No cache information exposed to the guest | libvirt mode='disable' |
The guest loses cache reporting; physical cache use is unchanged. |
| A fixed share of real host cache | Not provided by the documented libvirt cache element | Requires a separate hardware/host resource-partitioning facility, if the platform supports one; do not substitute cache-reporting XML. |
Bottom line for the common error
If your XML tries to assign a numeric cache size, replace that assumption with a choice between emulate, passthrough, and disable. Use QEMU’s smp-cache only after confirming local support, and treat it as cache-topology configuration rather than a guaranteed libvirt mechanism for allocating a specific number of megabytes. The exact error, complete XML, versions, architecture, machine type, and CPU mode are required to identify any remaining failure.
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