Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <CAKwvOd=jmi_7LvZbS0PA5-zt62m5ZFPUNG7JtMQVmerCR63ebQ@mail.gmail.com>
 <BDEF22F0-84A5-4A36-B153-13E6A4260922@zytor.com> <CAKwvOdnLt5qr_5ah86qdsocTxUbo_3X1y3PPRp1EZwt=__NxnA@mail.gmail.com>
 <dd1626ae-114f-3817-c8b2-f845e079bfc6@zytor.com>
In-Reply-To: <dd1626ae-114f-3817-c8b2-f845e079bfc6@zytor.com>
From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Thu, 24 May 2018 15:31:05 -0700
Message-ID: <CAKwvOdkEHqv7Ut8H6Rb+gSvfWsMHqO=E3f7yxm=06m9yEn=h6w@mail.gmail.com>
Subject: Re: [clang] stack protector and f1f029c7bf
To:     hpa@zytor.com
Cc:     Alistair Strachan <astrachan@google.com>,
        Manoj Gupta <manojgupta@google.com>,
        Matthias Kaehlcke <mka@google.com>,
        Greg Hackmann <ghackmann@google.com>, sedat.dilek@gmail.com,
        tstellar@redhat.com, LKML <linux-kernel@vger.kernel.org>,
        Kees Cook <keescook@google.com>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, May 24, 2018 at 3:05 PM H. Peter Anvin <hpa@zytor.com> wrote:
> COMPILER AR: "=rm" should NEVER generate worse code than "=r". That is
> unequivocally a compiler bug.

Filed: https://bugs.llvm.org/show_bug.cgi?id=37583

> >> You are claiming it doesn't buy us anything, but you are only looking
at
> > the paravirt case which is kind of "special" (in the short bus kind of
way),
> >
> > That's fair.  Is another possible solution to have paravirt maybe not
use
> > native_save_fl() then, but its own
non-static-inline-without-m-constraint
> > implementation?

> KERNEL AR: change native_save_fl() to an extern inline with an assembly
> out-of-line implementation, to satisfy the paravirt requirement that no
> GPRs other than %rax are clobbered.

i'm happy to add that, do you have a recommendation if it should go in an
existing .S file or a new one (and if so where/what shall I call it?).
-- 
Thanks,
~Nick Desaulniers