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Subject: Re: [PATCH 1/4] glibc: Perform rseq(2) registration at C startup and
 thread creation (v7)
To:     Michael Ellerman <mpe@ellerman.id.au>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Paul Burton <paul.burton@mips.com>,
        Will Deacon <will.deacon@arm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Russell King <linux@armlinux.org.uk>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Cc:     carlos <carlos@redhat.com>, Florian Weimer <fweimer@redhat.com>,
        Joseph Myers <joseph@codesourcery.com>,
        Szabolcs Nagy <szabolcs.nagy@arm.com>,
        libc-alpha <libc-alpha@sourceware.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ben Maurer <bmaurer@fb.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
        Rich Felker <dalias@libc.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>
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 <87lg0tosfz.fsf@concordia.ellerman.id.au>
From:   Carlos O'Donell <codonell@redhat.com>
Message-ID: <7412087f-5ef4-0670-503e-f8de6ca3b0bb@redhat.com>
Date:   Thu, 4 Apr 2019 16:15:53 -0400
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On 4/2/19 2:02 AM, Michael Ellerman wrote:
> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> writes:
>> Hi Carlos,
>>
>> ----- On Mar 22, 2019, at 4:09 PM, Carlos O'Donell codonell@redhat.com wrote:
> ...
>>
>> [...]
>>>> +++ b/sysdeps/unix/sysv/linux/powerpc/bits/rseq.h
>> [...]
>>>> +/* Signature required before each abort handler code.  */
>>>> +#define RSEQ_SIG 0x53053053
>>>
>>> Why isn't this an opcode specific to power?
>>
>> On powerpc 32/64, the abort is placed in a __rseq_failure executable section:
>>
>> #define RSEQ_ASM_DEFINE_ABORT(label, abort_label)                               \
>>                  ".pushsection __rseq_failure, \"ax\"\n\t"                       \
>>                  ".long " __rseq_str(RSEQ_SIG) "\n\t"                            \
>>                  __rseq_str(label) ":\n\t"                                       \
>>                  "b %l[" __rseq_str(abort_label) "]\n\t"                         \
>>                  ".popsection\n\t"
>>
>> That section only contains snippets of those trampolines. Arguably, it would be
>> good if disassemblers could find valid instructions there. Boqun Feng could perhaps
>> shed some light on this signature choice ? Now would be a good time to decide
>> once and for all whether a valid instruction would be a better choice.
> 
> I'm a bit vague on what we're trying to do here.
> 
> But it seems like you want some sort of "eye catcher" prior to the branch?
> 
> That value is a valid instruction on current CPUs (rlwimi.
> r5,r24,6,1,9), and even if it wasn't it could become one in future.
> 
> If you change it to 0x8053530 that is both a valid instruction and is a
> nop (conditional trap immediate but with no conditions set).

The s390 IBM team needs to respond to this and I want to make sure they ACK
the NOP being used here because it impacts them directly.

I'd like to see Martin's opinion on this.

-- 
Cheers,
Carlos.