From mboxrd@z Thu Jan  1 00:00:00 1970
From: Carlos O'Donell <codonell@redhat.com>
Subject: Re: [PATCH 1/4] glibc: Perform rseq(2) registration at C startup and
 thread creation (v7)
Date: Thu, 4 Apr 2019 16:32:53 -0400
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To: Florian Weimer <fweimer@redhat.com>, Michael Ellerman <mpe@ellerman.id.au>
Cc: 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>, carlos <carlos@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@g>
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On 4/2/19 3:08 AM, Florian Weimer wrote:
> * Michael Ellerman:
> 
>> 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).
> 
> I think we need something that is very unlikely to appear in the
> instruction stream.  It's just a marker.  The instruction will never be
> executed, and it does not have to be a trap, either (I believe that a
> standard trap instruction would be a bad choice).

I assume you want to avoid a standard trap instruction because it would
be common, and so not meet the intent of the RSEQ_SIG choice as being something
that is *uncommon* right?

It is valuable that it be a trap, particularly for constant pools because
it means that a jump into the constant pool will trap.

-- 
Cheers,
Carlos.