On Wed, 2018-01-10 at 16:47 +0100, Andrea Arcangeli wrote: > On Wed, Jan 10, 2018 at 03:24:17PM +0000, David Woodhouse wrote: > > Since it achieves nothing¹ but to make userspace run slower, there's no > > need to write it again on returning to userspace. It will perform that > > function just fine without doing so. > > Ok, very glad we are on the same page now. > > Note that as far as I can tell there was no way to answer the above > question by reading the spec.
The spec does, I concede, leave something to be desired. This sentence in particular — and it really is a single sentence — caused me to throw my toys out of the pram and demand verbal explanations (which is perhaps the only reason I managed to work it out): "If IBRS is set, near returns and near indirect jumps/calls will not allow their speculative target address to be controlled by code that executed in a less privileged prediction mode before the IBRS mode was last written with a value of 1 or on another logical processor so long as all RSB entries from the previous less privileged prediction mode are overwritten" But it *does* have the words "before the IBRS mode was last written with a value of 1" in there somewhere... ;)
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