Re: [PATCH 6/6] s390: introduce execute-trampolines for branches

[Martin Schwidefsky <schwidefsky@de.ibm.com>]

On Wed, 07 Feb 2018 12:07:55 +0000
David Woodhouse <dwmw2@infradead.org> wrote:

> On Wed, 2018-02-07 at 11:07 +0100, Pavel Machek wrote:
> > This is really unfortunate naming of kernel option.
> > 
> > spectre_v2=off sounds like we are turning the "bug" off, but i somehow
> > suspect you are turning the bug _workaround_ off.  
> 
> That's consistent with what we have on x86.
> 
> > > +ifdef CONFIG_EXPOLINE
> > > +  ifeq ($(call cc-option-yn,$(CC_FLAGS_MARCH) -mindirect-branch=thunk),y)
> > > +    CC_FLAGS_EXPOLINE := -mindirect-branch=thunk
> > > +    CC_FLAGS_EXPOLINE += -mfunction-return=thunk
> > > +    CC_FLAGS_EXPOLINE += -mindirect-branch-table
> > > +    export CC_FLAGS_EXPOLINE
> > > +    cflags-y += $(CC_FLAGS_EXPOLINE)
> > > +  else
> > > +    $(warning "Your gcc lacks the -mindirect-branch= option")
> > > +  endif
> > > +endif  
> 
> That isn't, though. Linus asked us to drop the $(warning) part.
> 
> ... and then spent a week building with a non-retpoline compiler and
> not noticing, so he might have changed his mind ;)

I found the warning to have some value, it helps for the case where my
fingers are faster than my brain and I type "make" instead of "smake"
which uses the alternative compiler with the required support.

@Linus: do you want a warning or prefer not to have one ?

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.