[$] Safer speculation-free user-space access

The Spectre class of hardware vulnerabilities truly is a gift that keeps on
giving. New variants are still being discovered in current CPUs nearly
eight years after the disclosure of this
problem, and developers are still working to minimize the performance costs
that come from defending against it. The masked user-space access
mechanism is a case in point: it reduces the cost of defending against some
speculative attacks, but it brought some challenges of its own that are
only now being addressed.