linux-kernel - Re: [PATCH v2 05/36] x86/entry/64/compat: Fix SYSENTER's NT flag before user memory access

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Message-ID: <20151007150509.GA23710@nazgul.tnic>
Date:	Wed, 7 Oct 2015 17:05:09 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Andy Lutomirski <luto@...nel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 05/36] x86/entry/64/compat: Fix SYSENTER's NT flag
 before user memory access

On Wed, Oct 07, 2015 at 10:33:10AM -0400, Brian Gerst wrote:
> This is an optimization, since NT is unlikely to be set.  The forward
> branch is usually predicted as not taken, and it keeps these rarely
> used instructions out of the hot cache.

Ah, slick. Because branch predictors mark new branches as not taken and
this particular one will remain non-taken in the majority of the cases.
I think this warrants a mention in the comment above it though...

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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