linux-kernel - RE: [PATCH] ARC: ARCv2: jump label: implement jump label patching

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Date:   Fri, 21 Jun 2019 15:39:27 +0000
From:   Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:     Vineet Gupta <Vineet.Gupta1@...opsys.com>
CC:     "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jason Baron <jbaron@...mai.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
Subject: RE: [PATCH] ARC: ARCv2: jump label: implement jump label patching

Hi Vineet,

> -----Original Message-----
> From: linux-snps-arc <linux-snps-arc-bounces@...ts.infradead.org> On Behalf Of Vineet Gupta
> Sent: Thursday, June 20, 2019 11:50 PM
> To: Peter Zijlstra <peterz@...radead.org>
> Cc: linux-arch@...r.kernel.org; Ard Biesheuvel <ard.biesheuvel@...aro.org>; Alexey Brodkin
> <abrodkin@...opsys.com>; linux-kernel@...r.kernel.org; Jason Baron <jbaron@...mai.com>; Paolo Bonzini
> <pbonzini@...hat.com>; linux-snps-arc@...ts.infradead.org; Eugeniy Paltsev
> <Eugeniy.Paltsev@...opsys.com>
> Subject: Re: [PATCH] ARC: ARCv2: jump label: implement jump label patching

[snip]

> Insn encoding is always middl eendina - irrespective of endianness.

Apparently only in little-endian mode instructions are encoded as middle-endian,
see:
-------------->8-------------
# cat endian.S

.global myfunc
myfunc:
        mov r0, r1
-------------->8-------------

Little-endian:
-------------->8-------------
# arc-linux-gcc -c -mcpu=archs endian.S -EL
# arc-linux-objdump -d endian.o

endian.o:     file format elf32-littlearc

Disassembly of section .text:
00000000 <myfunc>:
   0:   200a 0040               mov     r0,r1

# arc-linux-readelf -x .text endian.o
Hex dump of section '.text':
  0x00000000 0a204000                            . @.
-------------->8-------------

Big-endian:
-------------->8-------------
# arc-linux-gcc -c -mcpu=archs endian.S -EB
# arc-linux-objdump -d endian.o

endian.o:     file format elf32-bigarc

Disassembly of section .text:
00000000 <myfunc>:
   0:   200a 0040               mov     r0,r1

# arc-linux-readelf -x .text endian.o

Hex dump of section '.text':
  0x00000000 200a0040                             ..@
-------------->8-------------

-Alexey