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

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Message-ID: <CA+55aFwHuaj80NaRNZW3Jfzh7_=nsx5p5=07PXUpaYwLzH-T=Q@mail.gmail.com>
Date:	Wed, 25 Mar 2015 16:51:50 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	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

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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