A CPU read of GPU-written guest memory recorded a copy at the tail of
the current command buffer and waited for it, so it also waited for
every draw recorded after the writer. In Silent Hill's title menu most
of those reads targeted buffers whose writer had retired long before,
and UE's render thread spent a third of its time in them.
On a device with device-local, host-visible, coherent memory (Apple
GPUs), guest buffers are now allocated there. A readback whose writers
retired copies straight out of the buffer's mapping; one whose writer
is submitted but still running waits for that submission alone and then
copies; the command worker submits a writer that is still recording
and reads after it. A guest-thread wait also marks its window hot, so
the next GPU write there is submitted right after its command.
SHARPEMU_UNIFIED_BUFFERS=0 keeps device-local buffers.
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