Qemu is only able to achieve native performance when running in conjunction with a hypervisor like KVM. Hypervisors don't do binary translation, so the guest architecture needs to match the host architecture. Running x86_64 qemu under rosetta would likely be much slower than running aarch64 qemu, because it would be running an emulator inside of an emulator.

From the point of view of rosetta, qemu's JIT would be completely opaque, and so would end up suffering severe performance penalties due to it having to translate code from what would appear to be an aggressively self modifying JIT.

That said, assuming Qemu runs entirely in user space I would expect it to be able to run under rosetta, and am genuinely curious if it does, and what the perf is - as I said, I would expect it to be much slower than arm64 qemu emulating x86_64, but I'm curious as to how much.

Efficient use of qemu on x86 requires the Hypervisor framework, which isn't available under Rosetta.

It's possible to run qemu without Hypervisor.framework, but that means it's doing its own second layer of translation. This would be horribly inefficient under Rosetta.