From mboxrd@z Thu Jan  1 00:00:00 1970
From: Alan Modra <amodra@gmail.com>
Subject: Re: [PATCH 1/4] glibc: Perform rseq(2) registration at C startup and
 thread creation (v7)
Date: Tue, 9 Apr 2019 18:59:48 +0930
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On Tue, Apr 09, 2019 at 02:23:53PM +1000, Michael Ellerman wrote:
> I'd much rather we use a trap with a specific immediate value. Otherwise
> someone's going to waste time one day puzzling over why userspace is
> doing mtmsr.

It's data.  We have other data in executable sections.  Anyone who
wonders about odd disassembly just hasn't realized they are
disassembling data.

> It would also complicate things if we ever wanted to emulate mtmsr.

No, because it won't be executed.  If I understand correctly, the only
reason to choose an illegal, trap or privileged insn is to halt
execution earlier rather than later when a program goes off in the
weeds.

> If we want something that is a trap rather than a nop then use 0x0fe50553.
> 
> That's "compare the value in r5 with 0x553 and then trap unconditionally".
> 
> It shows up in objdump as:
> 
>     10000000:	53 05 e5 0f 	twui    r5,1363
> 
> 
> The immediate can be anything, I chose that value to mimic the x86 value
> Mathieu mentioned.
> 
> There's no reason that instruction would ever be generated because the
> immediate value serves no purpose. So it satisfies the "very unlikely
> to appear" criteria AFAICS.

Yes, looks fine to me, except that in VLE mode (do we care?)
".long 0x0fe50553" disassembles as
   0:	0f e5       	se_cmphl r5,r30
   2:	05 53       	se_mullw r3,r5
No illegal/trap/privileged insn there.

".long 0x0fe5000b" might be better to cover VLE.

-- 
Alan Modra
Australia Development Lab, IBM