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

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Message-ID: <YZeF4JjWIcTMtaaT@osiris>
Date:   Fri, 19 Nov 2021 12:09:20 +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 11:57:05AM +0100, Christian Borntraeger wrote:
> > > > > -                       "       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?

.short bypasses all sanity checks while .insn does not, so I think
that should be preferred. But I don't care too much.