RE: [patch RFC 5/5] x86/speculation: Add basic speculation control code

[Van De Ven, Arjan]

> On Wed, Jan 10, 2018 at 12:12:53PM +0000, David Woodhouse wrote:
> > IBRS is like a barrier. You must write it between the 'problematic'
> > loading of the branch targets, and the kernel code which might be
> > affected.
> >
> > You cannot, on current hardware, merely set it once and forget about
> > it. That is not sufficient.
> 
> I think you've got it all wrong...

Andrea: David is right. The specification draft that you have also makes this clear.

You can't just set IBRS once and call it good; you do need to write it on entering the kernel.