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

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Date:	Wed, 25 Mar 2015 16:56:00 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Denys Vlasenko <dvlasenk@...hat.com>
CC:	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	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

No, movabs is yet another instruction (with a 64-bit absolute address.) But movq can mean 10 or 7 bytes...

On March 25, 2015 4:51:50 PM PDT, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>On Wed, Mar 25, 2015 at 10:18 AM, Denys Vlasenko <dvlasenk@...hat.com>
>wrote:
>> The $AUDIT_ARCH_X86_64 parameter to syscall_trace_enter_phase1/2
>> is a 32-bit constant, loading it with 32-bit MOV produces 5-byte insn
>> instead of 10-byte one.
>
>Side note: has anybody talked to the assembler people? This would seem
>to be very much something that the assembler could have noticed and
>done on its own. It's a bit sad that we need to overspecify these
>things..
>
>If it had actually been a 64-bit constant, the assembler would have
>ended up silently using a different instruction encoding *anyway*
>("movabs"), so it's not like the "movq" in any way specifies one
>particular instruction representation, and the assembler already picks
>different instruction versions for different constant values. Why not
>this one?
>
>                       Linus

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