Intel Analysis of Speculative Execution Side Channels [pdf]
I was surprised that this wasn't already submitted -- and then surprised again that it's not being upvoted.  To give some color:  this doc contains the first public disclosure of some new MSRs being added to allow system software to help mitigate Spectre. In particular, these are the controls to limit speculation including Indirect Branch Restricted Speculation (IBRS) to restrict speculation of indirect branches; Single Thread Indirect Branch Predictors (STIBP) to prevent indirect branch predictions from being controlled by the sibling hyperthread (!!) and Indirect Branch Predictor Barrier (IBPB) to limit influence on later indirect branch predictions.  For those who need to implement system software support for the new microcode, this is very important information!