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Date: Tue, 10 Mar 2015 08:57:43 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, 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@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] x86: entry_32.S: change ESPFIX test to not touch
 PT_OLDSS(%esp)
Message-ID: <20150310075743.GA20041@gmail.com>
References: <1425919450-19116-1-git-send-email-dvlasenk@redhat.com>
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* Denys Vlasenko <dvlasenk@redhat.com> wrote:

> Old code was trying to avoid having three branch insns,
> but instead it has a chain of six insns where each insn
> depends on previos one.
> 
> And it was touching PT_OLDSS(%esp) unconditionally, even when it may
> contain bogus data. Elsewhere we have to jump thru hoops
> just to make sure here PT_OLDSS(%esp) is at least in a valid page.
> 
> All this just to have one branch instead of three?
> 
> The new code simply checks each condition.
> All three checks can run in parallel on an out-of-order CPU.
> Most of the time, none of branches will be taken.
> 
> Comparison of object code:
>     Old:
>      1e6:   8b 44 24 38             mov    0x38(%esp),%eax
>      1ea:   8a 64 24 40             mov    0x40(%esp),%ah
>      1ee:   8a 44 24 34             mov    0x34(%esp),%al
>      1f2:   25 03 04 02 00          and    $0x20403,%eax
>      1f7:   3d 03 04 00 00          cmp    $0x403,%eax
>      1fc:   74 0f                   je     20d <ldt_ss>
>     New:
>      1e6:   f6 44 24 3a 02          testb  $0x2,0x3a(%esp)
>      1eb:   75 0e                   jne    1fb <restore_nocheck>
>      1ed:   f6 44 24 34 03          testb  $0x3,0x34(%esp)
>      1f2:   74 07                   je     1fb <restore_nocheck>
>      1f4:   f6 44 24 40 04          testb  $0x4,0x40(%esp)
>      1f9:   75 0f                   jne    20a <ldt_ss>

Please do some benchmarking of this: a tight loop of getpid or getppid 
syscalls ought to be enough to be able to time this accurately.

Thanks,

	Ingo