linux-kernel - Re: [PATCH RESEND] x86/asm/entry/32: simplify pushes of zeroed pt

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Message-ID: <CALCETrVyzcD-Uwu25iLUh3_7s=vfi5oXC_y=R48zUgziMksNsg@mail.gmail.com>
Date:	Mon, 2 May 2016 11:19:18 -0700
From:	Andy Lutomirski <luto@...nel.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>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs

On Mon, May 2, 2016 at 7:56 AM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
> Use of a temporary R8 register here seems to be unnecessary.
>
> "push %r8" is a two-byte insn (it needs REX prefix to specify R8),
> "push $0" is two-byte too. It seems just using the latter would be
> no worse.
>
> Thus, code had an unnecessary "xorq %r8,%r8" insn.
> It probably costs nothing in execution time here since we are probably
> limited by store bandwidth at this point, but still.
>
> Run-tested under QEMU: 32-bit calls still work:

Acked-by: Andy Lutomirski <luto@...nel.org>