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

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Date:   Fri, 19 Nov 2021 11:54:41 +0100
From:   Heiko Carstens <hca@...ux.ibm.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Ilie Halip <ilie.halip@...il.com>,
        linux-kernel@...r.kernel.org, Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Mete Durlu <meted@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        linux-s390@...r.kernel.org, llvm@...ts.linux.dev,
        Ulrich Weigand <Ulrich.Weigand@...ibm.com>
Subject: Re: [PATCH] s390/test_unwind: use raw opcode instead of invalid
 instruction

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@...ibm.com>
> > Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.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.