Message-ID: <54ED005E.4070603@zytor.com>
Date: Tue, 24 Feb 2015 14:51:10 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Borislav Petkov <bp@alien8.de>, Denys Vlasenko <vda.linux@googlemail.com>
CC: Denys Vlasenko <dvlasenk@redhat.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@kernel.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>, X86 ML <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns
References: <1424803895-4420-1-git-send-email-dvlasenk@redhat.com> <20150224195801.GM3420@pd.tnic> <CAK1hOcMcnTGyq9mJ_iOCq29-q54YaNzKxNb05Z29wdkX1X6+Og@mail.gmail.com> <20150224203635.GN3420@pd.tnic>
In-Reply-To: <20150224203635.GN3420@pd.tnic>
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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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