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

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Message-ID: <20150326093053.GB14706@gmail.com>
Date:	Thu, 26 Mar 2015 10:30:53 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	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


* 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. [...]

Maybe GCC already picks a 32-bit opcode in these small-constant cases, 
so there was little incentive to optimize on the GAS side, other than 
making it correct.

> [...] It's a bit sad that we need to overspecify these things..

Yeah, that's sad.

Yesterday when I have read Denys's patch I double checked that there's 
no other similar (easily identifiable ...) movq opcode left in the 
64-bit entry code, so we seem to have squashed most of them.

Thanks,

	Ingo
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