The out of bounds variant is fixable in the OS, just add a fence instruction between the check and the load.

For code running in user mode, you flush the branch predictor on each context switch---again, new microcode + patched OS.

Hyperthreads are tricky. Those are not yet fixed by microcode AIUI, and in the future you may want a usermode program to say "I don't want indirect branch prediction because I am afraid of what the other hyperthread might do to me". That would require some new system call (like a new prctl on Linux) or something like that.

Great. Now we just have to think of new attacks using the same general idea to slow down all computers by yet another 10% :p