linux-kernel - Re: [PATCH 5/5] x86/asm/entry/32: Simplify FLAGS

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Date:	Mon, 27 Jul 2015 15:40:00 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Krzysztof A. Sobiecki" <sobkas@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, 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>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] x86/asm/entry/32: Simplify FLAGS_NT clearing in
 SYSENTER32 code.

On Mon, Jul 27, 2015 at 1:33 PM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
> "sysenter_fix_flags" detour does not need to be convoluted anymore,
> straigten it up. However, we still use this:
>
>         jnz     2f
>         jmp     sysenter_jumps_here
> 2:      ...
>
> instead of this:
>
>         jz      sysenter_jumps_here
>         ...
>
> because "cold" conditional forward branch is predicted not taken
> by most CPUs - exactly what we want. Latter version would get it wrong.
>
> Reinstate "why we use SYSRETL instead of SYSEXIT" comment.
>

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