> > >> From: Sai Praneeth <sai.praneeth.prak...@intel.com> Some future > > >> Intel processors may support "Enhanced IBRS" which is an "always > > >> on" mode i.e. IBRS bit in SPEC_CTRL MSR is enabled once and never > > >> disabled. According to specification[1], this should simplify > > >> software enabling and improve performance. > > > SHOULD is not really helpful. The question is whether it does > > > improve performance in practice or not. You really want to add > > > numbers comparing retpoutine and enhanced IBRS. > > > > One thing to remember from Intel's retpoline paper: > > > > > Retpoline is known to be an effective branch target injection > > > (Spectre variant 2) mitigation on Intel processors belonging to > > > family 6 (enumerated by the CPUID instruction) that do not have > > > support for enhanced IBRS. On processors that support enhanced IBRS, > > > it should be used for mitigation instead of retpoline. > > > > That's both a statement of "Intel would like you to use enhanced IBRS > > over retpoline where available" and "retpoline provides less > > mitigation on processors with enhanced IBRS compared to those without". > > > > In other words, we can _do_ performance deltas, but they won't be as > > meaningful because they won't really have apples-to-apples mitigation > > properties. > > Fair enough, but this wants to be spelled out in the change log explicitely > instead > of unspecific blurbs.
Sure! Makes sense. Would you like me to send a V2 with updated change log? Regards, Sai
