Re: [PATCH v3] arm64: sysreg: make mrs_s and msr_s macros work with Clang and LTO

[Nick Desaulniers]

On Tue, Apr 23, 2019 at 3:57 PM Ard Biesheuvel
<ard.biesheuvel@xxxxxxxxxx> wrote:
>
> On Wed, 24 Apr 2019 at 00:55, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >
> > On Mon, Apr 15, 2019 at 7:38 AM Kees Cook <keescook@xxxxxxxxxxxx> 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

-- 
Thanks,
~Nick Desaulniers