Messages in this thread | | | Date | Fri, 16 Mar 2018 10:15:49 +0100 | From | Michal Suchánek <> | Subject | Re: [PATCH RFC rebase 3/9] powerpc/64: Use barrier_nospec in syscall entry |
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Hello,
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.
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
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