Hi Alan, On Thu, Jan 04, 2018 at 05:04:42PM +0000, Alan Cox wrote: > > If you run lots of syscalls ibrs 1 ibpb 1 is much faster. If you do > > infrequent syscalls computing a lot in kernel like I/O with large > > buffers getting copied, ibrs 0 ibpb 2 is much faster than ibrs 1 ibpb > > 1 (on those microcodes where ibrs 1 reduces performance a lot, not all > > microcodes implementing SPEC_CTRL are inefficient like that). > > Have you looked at whether you can measure activity and switch > automatically between the two (or by task). It seems silly to leave > something the machine can accurately assess toa human ?
We didn't but it'd be definitely reasonable to investigate and it's a good idea for those CPUs where the updated microcode has to shutdown way more than just indirect branch prediction speculation to achieve the ibrs 1 semantics. If the workload changes from frequent syscalls to reasonably large read/writes and less frequent syscalls or lots of interrupts in idle CPUs, it would work well to switch between ibrs 1 ibpb 1 and ibpb 2 ibrs 0 automatically. As long as the pattern keeps repeating for a while... that is the question ;). Thanks! Andrea
