I can answer (2) only. On 10/01/2018 17:19, Liran Alon wrote: > (2) On VMExit, Intel recommends to always save guest SPEC_CTRL value, > set IBRS to 1 (even if it is already set by guest) and stuff RSB. What > exactly does this write of 1 to IBRS do? > * a) Does it keep all currently existing BTB/BHB entries created by > less-privileged prediction-mode and marks them as were created in > less-privileged prediction-mode such that they won't be used in current > prediction-mode? > * b) Or does it, as Paolo has mentioned multiple times, disables the > branch predictor to never consult the BTB/BHB at all as long as IBRS=1? > If (b) is true, why is setting IBRS=1 better than just issue an IBPB that clears all entries? At least in that case the > host kernel could still benefict branch prediction performance boost. Arjan said (b) is not true on all processor generations. But even if it were true, setting IBRS=1 is much, much faster than IBPB. > If (a) is true, does "IBRS ALL THE TIME" usage is basically a CPU > change to just create all BTB/BHB entries to be tagged with > prediction-mode at creation-time and that tag to be compared to current > prediction-mode when CPU attempts to use BTB/BHB? I hope so, and I hope said prediction mode includes PCID/VPID too. While I agree with David that "we have other things to work on for now before we support hypothetical future hardware", I'd like to make sure that Intel gets it right... Paolo
