[$] Safer speculation-free user-space access

The Spectre class of hardware vulnerabilities truly is a gift that keeps ongiving. New variants are still being discovered in current CPUs nearlyeight years after the disclosure of thisproblem, and developers are still working to minimize the performance coststhat come from defending against it. The masked user-space accessmechanism is a case in point: it reduces the cost of defending against somespeculative attacks, but it brought some challenges of its own that areonly now being addressed.