[$] Fighting Spectre with cache flushes

One of the more difficult aspects of the Spectre hardware vulnerability isfinding all of the locations in the code that might be exploitable. Thereare many locations that look vulnerable that aren't, and others that areexploitable without being obvious. It has long been clear that finding allof the exploitable spots is a long-term task, and keeping new ones frombeing introduced will not be easy. But there may be a simple technique thatcan block a large subset of the possible exploits with a minimal cost.