Yo. As I understand it, on intel cpus and possibly more, we'll need to unmap the kernel on userret, or else userland can read arbitrary kernel memory.
People seem to be mentioning a 50% performance penalty and we might do worse (we don't have vDSOs...) Also, I understand that to exploit this, one has to attempt to access kernel memory a lot, and SEGV at least once per bit. I wonder if we can count the number of SEGVs and if we get a few, turn on the workaround? that would at least spare us the performance penalty for normal code.
