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

[Michal SuchÃnek]

Hello,

On Fri, 16 Mar 2018 15:18:23 +1000
Nicholas Piggin <npiggin@xxxxxxxxx> wrote:

> On Thu, 15 Mar 2018 20:15:52 +0100
> Michal Suchanek <msuchanek@xxxxxxx> 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.

Thanks

Michal