Opt-In L1 Cache Flushing To Try For Linux 5.15 To Help With The Paranoid, Future CPU Vulnerabilities

Worked on for more than one year is the patches out of Amazon for allowing opt-in L1 data cache flushing on context switching. This L1d flushing is done in the name of greater security given the various CPU speculative execution hardware vulnerabilities these days and protecting against other possible future vulnerabilities. After trying to get the code merged last summer, Linus Torvalds called it "beyond stupid" and reverted the code but now for Linux 5.15 a revised form of it was submitted...