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Cloud Hypervisor maps guest RAM with MAP_NORESERVE, so the kernel never reserves the backing pages at mmap time. On a host whose hugepage pool cannot satisfy every guest, a VM is created successfully and then takes a SIGBUS when the guest faults a page the pool can no longer back. This is the failure mode reported in #5730 and #7387. As noted on #5730, checking free pool headroom up front is not a reliable fix: another process can consume pages between the check and the fault. Add a reserve=on parameter to --memory and --memory-zone (default off, preserving the current MAP_NORESERVE behaviour). When set, the region is mapped without MAP_NORESERVE, so the kernel reserves the backing pages (swap, or huge pages for hugepage-backed memory) at mmap time, atomically with the mapping. An over-committed configuration then fails cleanly at VM creation with an mmap ENOMEM instead of crashing the guest later. Unlike prefault it does not fault the memory in, so it does not slow down boot. This mirrors QEMU's memory-backend reserve property, which has the same name and meaning (reserve=off maps with MAP_NORESERVE). reserve is threaded through the same mmap paths as the existing prefault option, and is exposed in the OpenAPI schema, CLI help and docs. The top-level --memory reserve=on path is unchanged: the default zone is synthesised from MemoryConfig and inherits its reserve value. Assisted-by: Claude Code (Opus 4.8) Signed-off-by: Ian Klemm <hi@ianklemm.de>