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

[Heiko Carstens <hca@linux.ibm.com>]

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@de.ibm.com>
> > Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> 
> 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".

Plus a fixup of the comment above, since this would generate an
operation insteand of a specification exception. Just a generic
"exception" would be good enough for the comment.