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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Fri, 18 Oct 2019 10:05:18 -0700
Message-ID: <CABCJKud7bJOQqyve9=niSP62H0WTrCk5ZAmAcD2-KR=vf_gn0Q@mail.gmail.com>
Subject: Re: [PATCH 13/18] arm64: preserve x18 when CPU is suspended
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Dave Martin <Dave.Martin@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Laura Abbott <labbott@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Fri, Oct 18, 2019 at 9:49 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
> > diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> > index fdabf40a83c8..9a8bd4bc8549 100644
> > --- a/arch/arm64/mm/proc.S
> > +++ b/arch/arm64/mm/proc.S
> > @@ -73,6 +73,9 @@ alternative_endif
> >         stp     x8, x9, [x0, #48]
> >         stp     x10, x11, [x0, #64]
> >         stp     x12, x13, [x0, #80]
> > +#ifdef CONFIG_SHADOW_CALL_STACK
> > +       stp     x18, xzr, [x0, #96]
>
> Could this be a str/ldr of just x18 rather than stp/ldp of x18 +
> garbage?  Maybe there's no real cost difference, or some kind of
> alignment invariant?

Sure, this can be changed to str/ldr. I don't think there's a
noticeable difference in cost.

Sami