linux-kernel - Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns

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Message-ID: <54ED005E.4070603@zytor.com>
Date:	Tue, 24 Feb 2015 14:51:10 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...en8.de>,
	Denys Vlasenko <vda.linux@...glemail.com>
CC:	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	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 Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns

On 02/24/2015 12:36 PM, Borislav Petkov wrote:
> On Tue, Feb 24, 2015 at 09:13:03PM +0100, Denys Vlasenko wrote:
>> They aren't equal.  $1 and $2 in two lowest bits will also
>> be interpreted as "userspace" here. "Equal to $3" sends
>> a wrong message here to a human reading the code,
>> the code doesn't test for CPL=3, it tests for any nonzero CPL.
> 
> Doh, of course.
> 
> I was going to propose to make it even more explicit:
> 
> 	andb $3, CS(%rsp)...
> 
> but that's destructive.
> 
> So yours makes it less misleading, actually. To me at least and
> obviously.
> 

Yes, please use JZ/JNZ with TEST.

	-hpa


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