Re: [PATCH RFC rebase 3/9] powerpc/64: Use barrier_nospec in syscall entry

[Michael Ellerman <mpe@ellerman.id.au>]

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