Bad idea in the wake of Spectre and meltdown. You want user code in a different address space for those reasons these days. Unless you mean more like using the

Bad idea in the wake of Spectre and meltdown. You want user code in a different address space for those reasons these days. Unless you mean more like using the user callback in the same way, say, a posix signal handler works, or Microsoft's SEH or APCs, then I would say it's already possible with a C ABI, citing those things as examples.