On Fri, 16 Mar 2018 10:15:49 +0100 Michal Suchánek <msucha...@suse.de> wrote:
> Hello, > > On Fri, 16 Mar 2018 15:18:23 +1000 > Nicholas Piggin <npig...@gmail.com> wrote: > > > On Thu, 15 Mar 2018 20:15:52 +0100 > > Michal Suchanek <msucha...@suse.de> wrote: > > > > > On powerpc syscall entry is done in assembly so patch in an explicit > > > barrier_nospec. > > > > Same comment as Linus for this -- the barriers are before the branch > > here, so is it possible the branch instruction can be speculative > > while the index is used to load the syscall table? > > As far as I understand barriers they separate code before the barrier > and code after the barrier. > > So inserting barrier_nospec after cmpldi means that the result of the > cmpldi has to be known before any instruction following barrier_nospec > that depends on the result can be executed. > > In many cases it is useful to put the barrier after a branch. It allows > the compiler to speculate on the computed value at compile time and if > it is constrained optimize out the branch. It may also result in the > need to include many barriers and less readable code. > > However, you have probably knowledge of the powerpc implementation of > the barrier so if the semantic is actually different then please > enlighten me. I actually don't. I'm assuming we should be able to say that no previous instruction is speculative when a subsequent one is executed. But the branch instruction itself that is speculated, not the compare. Usually even if all sources are ready, the pipeline may take in some cycles after a branch, before that branch can finish executing and squash speculation if it was wrong. Perhaps there is only a couple of cycles of instructions that get a chance to reach execution units and disturb any caches, but still there could be some window and I don't think we would have architectural gurantees on that. I'll try to ask around and see if there's any documentation we can give you yet. Thanks, Nick
