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

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Message-Id: <20180207131719.4aeb316e@mschwideX1>
Date:   Wed, 7 Feb 2018 13:17:19 +0100
From:   Martin Schwidefsky <schwidefsky@...ibm.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jon Masters <jcm@...hat.com>,
        Marcus Meissner <meissner@...e.de>,
        Jiri Kosina <jkosina@...e.cz>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>
Subject: Re: [PATCH 6/6] s390: introduce execute-trampolines for branches

On Wed, 07 Feb 2018 12:07:55 +0000
David Woodhouse <dwmw2@...radead.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.