Guest memcpy went through the managed gateway on every call: two write
tracking notifications, the memory reader lock and two region lookups, about
0.8 us per call under Rosetta even for a copy of a few bytes. Dreaming Sarah
makes roughly 45k of those calls a second.
Guest memory is identity-mapped, and every fault a copy can take -- write
tracking, lazy commit -- is resolved from the fault address alone, so the copy
can run as a forward rep movsb in a fast-path stub. A destination that sits
inside the source, where a forward copy would read bytes it has already
written, and the write-watch diagnostic both keep the managed export.
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