linux-kernel - Re: [PATCH v4 03/17] arm64: kvm: stop treating register x18 as caller save

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Date:   Mon, 4 Nov 2019 13:44:30 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 03/17] arm64: kvm: stop treating register x18 as caller save

On Mon, Nov 4, 2019 at 3:51 AM Mark Rutland <mark.rutland@....com> wrote:
> > --- a/arch/arm64/kvm/hyp/entry.S
> > +++ b/arch/arm64/kvm/hyp/entry.S
> > @@ -23,6 +23,7 @@
> >       .pushsection    .hyp.text, "ax"
> >
>
> Could we please add a note here, e.g.
>
> /*
>  * We treat x18 as callee-saved as the host may use it as a platform
>  * register (e.g. for shadow call stack).
>  */
>
> ... as that will avoid anyone trying to optimize this away in future
> after reading the AAPCS.

Sure, that's a good idea.

> >  .macro restore_callee_saved_regs ctxt
> > +     // We assume \ctxt is not x18-x28
>
> Probably worth s/assume/require/ here.

Agreed, I'll change this in v5.

Sami