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Date:   Thu, 18 Apr 2019 11:31:56 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Alan Modra <amodra@gmail.com>
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Subject: Re: [PATCH 1/4] glibc: Perform rseq(2) registration at C startup
 and thread creation (v7)
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----- On Apr 9, 2019, at 5:29 AM, Alan Modra amodra@gmail.com wrote:

> 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.

Can you share with us the objdump output of ".long 0x0fe5000b" in
VLE mode ? VLE mode support does not appear to be available in typical
toolchains. Also, is VLE mode only for powerpc 32 be, or also for
powerpc 64 be/le ?

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com