Re: [PATCH] s390/test_unwind: use raw opcode instead of invalid instruction

[Christian Borntraeger]

Am 19.11.21 um 11:54 schrieb Heiko Carstens:
> On Fri, Nov 19, 2021 at 10:39:15AM +0100, Christian Borntraeger wrote:
> > > So if I understand
> > > https://sourceware.org/binutils/docs/as/s390-Directives.html#s390-Directives
> > > https://sourceware.org/binutils/docs/as/s390-Formats.html
> > > that `e,` prefix is for 16B opcodes?
> >
> > e is an instruction format as specified by the architecture.
> > See http://publibfp.dhe.ibm.com/epubs/pdf/a227832c.pdf
> > without any parameters.
> > Normally RR would be the right thing for MVCL, but since
> > we try to build an invalid opcode without the assembler
> > noticing (ab)using e seem like a safer approach.
> > > LGTM, thanks again.
> > > Suggested-by: Ulrich Weigand <Ulrich.Weigand@xxxxxxxxxx>
> > > Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> >
> > added those and added my RB. applied to the s390 tree. Thanks
> ..
> > > > diff --git a/arch/s390/lib/test_unwind.c b/arch/s390/lib/test_unwind.c
> > > > index cfc5f5557c06..d342bc884b94 100644
> > > > --- a/arch/s390/lib/test_unwind.c
> > > > +++ b/arch/s390/lib/test_unwind.c
> > > > @@ -176,7 +176,7 @@ static noinline int unwindme_func4(struct unwindme *u)
> > > >                    * trigger specification exception
> > > >                    */
> > > >                   asm volatile(
> > > > -                       "       mvcl    %%r1,%%r1\n"
> > > > +                       "       .insn e,0x0e11\n"       /* mvcl %%r1,%%r1" */
>
> Sorry, I disagree with this. As you said above rr would be the correct
> format for this instruction. If we go for the e format then we should
> also use an instruction with e format.
> Which in this case would simply be an illegal opcode, which would be
> sufficient for what this code is good for: ".insn e,0x0000".

Why not simply use .short then?