Messages in this thread | | | From | Michael Ellerman <> | Subject | Re: [PATCH RFC rebase 3/9] powerpc/64: Use barrier_nospec in syscall entry | Date | Sat, 17 Mar 2018 00:28:42 +1100 |
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Hi Michal,
Thanks for working on this series in the absence of any documentation.
Michal Suchánek <msuchanek@suse.de> writes: > On Fri, 16 Mar 2018 15:18:23 +1000 > Nicholas Piggin <npiggin@gmail.com> wrote: > >> On Thu, 15 Mar 2018 20:15:52 +0100 >> Michal Suchanek <msuchanek@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.
That would make sense, but I don't think that's how the barrier's been defined.
I don't have a formal spec for it (yet), but what I do have indicates it only orders older branches vs future instructions.
> However, you have probably knowledge of the powerpc implementation of > the barrier so if the semantic is actually different then please > enlighten me.
We have some knowledge, but only some :)
It's not necessarily implemented the same way on each chip revision, so it's not entirely clear what the formal semantics will be vs what we are seeing in current implementations. But I think it's safe to say it should always go after the branch that might be speculatively executed.
Will try and get some better documentation for you.
cheers
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