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

On Fri, 16 Mar 2018 10:15:49 +0100
Michal Suchánek [off-list ref] wrote:

Hello,

On Fri, 16 Mar 2018 15:18:23 +1000
Nicholas Piggin [off-list ref] wrote:

quoted

On Thu, 15 Mar 2018 20:15:52 +0100
Michal Suchanek [off-list ref] wrote:
  

quoted

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