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Date:   Thu, 18 Apr 2019 14:58:20 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Paul Burton <paul.burton@mips.com>
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        Paul Mackerras <paulus@samba.org>,
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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 4, 2019, at 5:41 PM, Paul Burton paul.burton@mips.com wrote:
[...]

>> 2a. A uncommon TRAP hopefully with some immediate data encoded (maybe uncommon)
> 
> Our break instruction has a 19b immediate in nanoMIPS (20b for microMIPS
> & classic MIPS) so that could be something like:
> 
>  break 0x7273       # ASCII 'rs'
> 

Hi Paul,

I like this uncommon break instruction as signature choice.

However, if I try to compile assembler with a break 0x7273 instruction
with mips64 and mips32 toolchains (gcc version 8.2.0 (Ubuntu 8.2.0-1ubuntu2~18.04))
I get:

/tmp/ccVh9F7T.s: Assembler messages:
/tmp/ccVh9F7T.s:24: Error: operand 1 out of range `break 0x7273'

It works up to the value 0x3FF, which seems to use the top 10
code bits:

   a:	03ff 0007 	break	0x3ff

Would a "break 0x350" be a good choice as well ? 

Any idea why 0x7273 is not accepted by my assembler ?

I also tried crafting the assembler with values between 0x3FF and 0x7273
in the 20 code bits. It seems fine from an objdump perspective:

".long 0x03FFFC7\n\t"

generates:

  10:	003f ffc7 	break	0x3f,0x3ff

What I don't understand is why the instruction generated by my
toolchain ends with the last 6 bits "000111", whereas the mips32
instruction set specifies break as ending with "001101" [1].
What am I missing ?

Also, the nanomips break code [2] has a completely different
instruction layout. Should we use a different signature when
compiling for nanomips ? What #ifdef should we use ? Do I
need a special toolchain to generate nanomips binaries ?

Thanks,

Mathieu

[1] http://hades.mech.northwestern.edu/images/1/16/MIPS32_Architecture_Volume_II-A_Instruction_Set.pdf
[2] https://s3-eu-west-1.amazonaws.com/downloads-mips/I7200/I7200+product+launch/MIPS_nanomips32_ISA_TRM_01_01_MD01247.pdf

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