Message-ID: <658a63b5-2d18-2837-9639-75a14c959f73@de.ibm.com>
Date:   Fri, 19 Nov 2021 11:57:05 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.2.0
Subject: Re: [PATCH] s390/test_unwind: use raw opcode instead of invalid
 instruction
Content-Language: en-US
To:     Heiko Carstens <hca@linux.ibm.com>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Ilie Halip <ilie.halip@gmail.com>,
        linux-kernel@vger.kernel.org, Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Mete Durlu <meted@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        linux-s390@vger.kernel.org, llvm@lists.linux.dev,
        Ulrich Weigand <Ulrich.Weigand@de.ibm.com>
References: <20211117174822.3632412-1-ilie.halip@gmail.com>
 <CAKwvOd=9tsHHhPBOx2ORZoJP09VsX5dRZn58qj3MzCc2vmVosg@mail.gmail.com>
 <d9ec2704-f41c-eafa-1945-ce845d65be8a@de.ibm.com> <YZeCcSjh4yCzzDcH@osiris>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
In-Reply-To: <YZeCcSjh4yCzzDcH@osiris>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0
Precedence: bulk


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@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".

Why not simply use .short then?