feat(vm): replace --on-demand with --restore-mode copy|ondemand|mmap#127
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CH's restore API takes one memory_restore_mode enum; a bool flag can only reach one of its three values. --restore-mode passes the value through (empty/copy omits the field, preserving the wire default), adding the new mmap mode: CoW-map the snapshot memory file so clones of one snapshot share page cache — measured on 16 cores: 16-way clone burst per-clone p50 450ms -> 79ms vs copy. ondemand/mmap require the snapshot file to remain on disk; mmap additionally requires it unchanged for the VM lifetime. --on-demand is removed rather than aliased: it mapped to the same field and CH-side gains nothing from a second spelling.
An older CH has no memory_restore_mode field and serde silently ignores it, restoring by copy while reporting success; say so instead of gating restore on a version probe the clone hot path would have to pay for.
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What
Replaces the bool
--on-demandflag onvm clone/vm restorewith--restore-mode copy|ondemand|mmap, passed straight through to Cloud Hypervisor'smemory_restore_moderestore field.--on-demandis removed rather than aliased: it mapped to the same single CH enum field, and a second spelling buys nothing.copyomits the field (wire behavior identical to before — old CH binaries unaffected).ondemand→OnDemand(UFFD lazy restore, unchanged semantics).mmap→Mmap(new CH mode, vmm: add mmap memory restore mode cloud-hypervisor#1): CoW-maps the snapshot memory file so clones of one snapshot share page cache. Measured on 16 cores: 16-way clone burst per-clone p50 450ms → 79ms vs copy.--on-demand(flag help says CH only).types.VMConfig.OnDemand bool→RestoreMode string, still transient (json:"-"), never persisted.Evidence
make lintdual-platform 0 issues;go test -race ./...28 pkgs ok; new table test for the flag validation.--restore-mode mmapclone boots + exec ✓, default copy ✓,--restore-mode ondemandclone via UFFD ✓, invalid value rejected at parse ✓,--on-demandnow unknown flag ✓,vm restore --restore-mode mmapafter stop ✓.Note:
mmap/ondemandrequire the snapshot file to remain on disk for the VM lifetime;mmapadditionally requires it unchanged (documented in the CH PR).