linux-kernel - Re: [PATCH 3/4] x86/asm/entry/64: use smaller insns

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Message-ID: <20150326102531.GA19907@gmail.com>
Date:	Thu, 26 Mar 2015 11:25:31 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] x86/asm/entry/64: use smaller insns


* Denys Vlasenko <vda.linux@...glemail.com> wrote:

> In my experiment, GAS uses 10-byte insn only for constants which
> won't work with 7-byte encoding; or if I explicitly ask for "movabs":
> 
> _start:         .globl  _start
>                 mov $0x12345678,%edi  # 5 bytes
>                 mov $0x12345678,%rdi  # 7 bytes
>                 movq $0x12345678,%rdi  # 7 bytes
>                 mov $0x80000000,%rdi  # 10 bytes
>                 mov $0x123456789,%rdi  # 10 bytes
>                 movabs $0x12345678,%rdi # 10 bytes
> 
> 
> $ gcc -nostartfiles -nostdlib -c z.S && objdump -dr z.o
> z.o:     file format elf64-x86-64
> Disassembly of section .text:
> 0000000000000000 <_start>:
>    0:    bf 78 56 34 12           mov    $0x12345678,%edi
>    5:    48 c7 c7 78 56 34 12     mov    $0x12345678,%rdi
>    c:    48 c7 c7 78 56 34 12     mov    $0x12345678,%rdi
>   13:    48 bf 00 00 00 80 00     movabs $0x80000000,%rdi
>   1a:    00 00 00
>   1d:    48 bf 89 67 45 23 01     movabs $0x123456789,%rdi
>   24:    00 00 00
>   27:    48 bf 78 56 34 12 00     movabs $0x12345678,%rdi
>   2e:    00 00 00

I see, so:

       movq $AUDIT_ARCH_X86_64, %rsi

generated a 10-byte MOVABS opcode, while moving into %esi generates 
the 5-byte 32-bit MOV opcode?

Thanks,

	Ingo
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