linux-kernel - Re: [PATCH v3] arm64: sysreg: make mrs_s and msr

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Message-ID: <CAGXu5j+9FFV-qoeXczhkTYiwuXcX+urmAU2eazk26XCdN476KA@mail.gmail.com>
Date:   Tue, 23 Apr 2019 16:09:05 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Alex Matveev <alxmtvv@...il.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Yury Norov <ynorov@...iumnetworks.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] arm64: sysreg: make mrs_s and msr_s macros work with
 Clang and LTO

On Tue, Apr 23, 2019 at 4:00 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Tue, Apr 23, 2019 at 3:57 PM Ard Biesheuvel
> <ard.biesheuvel@...aro.org> wrote:
> >
> > On Wed, 24 Apr 2019 at 00:55, Kees Cook <keescook@...omium.org> wrote:
> > >
> > > On Mon, Apr 15, 2019 at 7:38 AM Kees Cook <keescook@...omium.org> wrote:
> > > > +#define __msr_s(r, v)                                          \
> > > > +       DEFINE_MSR_S                                            \
> > > > +"      msr_s " __stringify(r) ", %x0\n"                        \
> > > > +       UNDEFINE_MSR_S : : "rZ" (v)
> > >
> > > BTW ... is "%x0" a typo here? Shouldn't this just be "%0"?
> > >
> >
> > No, iirc, that is required to force clang to choose a x# encoding
> > (which is the only one supported by msr/mrs instructions). Otherwise,
> > it will infer from the operand size which encoding to use, and attempt
> > to use w# for 32-bit system registers.
>
> Yes: https://github.com/llvm/llvm-project/blob/80b578c7325352187d0aff7bd2bdb5a76b51cf37/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp#L466

Cool, yeah, I see now. Thanks!

-- 
Kees Cook