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Message-ID: <1518006004.3677.115.camel@infradead.org>
Subject: Re: [PATCH 6/6] s390: introduce execute-trampolines for branches
From:   David Woodhouse <dwmw2@infradead.org>
To:     Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Pavel Machek <pavel@ucw.cz>, linux-kernel@vger.kernel.org,
        linux-s390@vger.kernel.org,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Cornelia Huck <cohuck@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jon Masters <jcm@redhat.com>,
        Marcus Meissner <meissner@suse.de>,
        Jiri Kosina <jkosina@suse.cz>,
        Dominik Brodowski <linux@dominikbrodowski.net>,
        Alan Cox <gnomes@lxorguk.ukuu.org.uk>
In-Reply-To: <20180207131719.4aeb316e@mschwideX1>
References: <1517986811-27819-1-git-send-email-schwidefsky@de.ibm.com>
         <1517986811-27819-7-git-send-email-schwidefsky@de.ibm.com>
         <20180207100726.GB31392@amd> <1518005275.3677.112.camel@infradead.org>
         <20180207131719.4aeb316e@mschwideX1>
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Date:   Wed, 07 Feb 2018 12:20:04 +0000
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On Wed, 2018-02-07 at 13:17 +0100, Martin Schwidefsky wrote:
> On Wed, 07 Feb 2018 12:07:55 +0000
> David Woodhouse <dwmw2@infradead.org> wrote:
>=20
> >=20
> > On Wed, 2018-02-07 at 11:07 +0100, Pavel Machek wrote:
> > >=20
> > > This is really unfortunate naming of kernel option.
> > >=20
> > > spectre_v2=3Doff sounds like we are turning the "bug" off, but i some=
how
> > > suspect you are turning the bug _workaround_ off.=C2=A0=C2=A0
> > That's consistent with what we have on x86.
> >=20
> > >=20
> > > >=20
> > > > +ifdef CONFIG_EXPOLINE
> > > > +=C2=A0=C2=A0ifeq ($(call cc-option-yn,$(CC_FLAGS_MARCH) -mindirect=
-branch=3Dthunk),y)
> > > > +=C2=A0=C2=A0=C2=A0=C2=A0CC_FLAGS_EXPOLINE :=3D -mindirect-branch=
=3Dthunk
> > > > +=C2=A0=C2=A0=C2=A0=C2=A0CC_FLAGS_EXPOLINE +=3D -mfunction-return=
=3Dthunk
> > > > +=C2=A0=C2=A0=C2=A0=C2=A0CC_FLAGS_EXPOLINE +=3D -mindirect-branch-t=
able
> > > > +=C2=A0=C2=A0=C2=A0=C2=A0export CC_FLAGS_EXPOLINE
> > > > +=C2=A0=C2=A0=C2=A0=C2=A0cflags-y +=3D $(CC_FLAGS_EXPOLINE)
> > > > +=C2=A0=C2=A0else
> > > > +=C2=A0=C2=A0=C2=A0=C2=A0$(warning "Your gcc lacks the -mindirect-b=
ranch=3D option")
> > > > +=C2=A0=C2=A0endif
> > > > +endif=C2=A0=C2=A0
> > That isn't, though. Linus asked us to drop the $(warning) part.
> >=20
> > ... 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.
>=20
> @Linus: do you want a warning or prefer not to have one ?

FWIW I agreed to drop it when the plan in my head was "we'll just turn
on IBRS instead if the compiler doesn't do full retpoline support".

Now that Linus has expressed a disinclination to take IBRS support in
that form, I might be more inclined to defend the $(warning) too.
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