linux-kernel - [tip:x86/asm] x86/entry/64/compat: Document sysenter_fix

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Date:	Sun, 11 Oct 2015 02:09:34 -0700
From:	tip-bot for Borislav Petkov <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	peterz@...radead.org, mingo@...nel.org, luto@...capital.net,
	brgerst@...il.com, luto@...nel.org, bp@...en8.de,
	dvlasenk@...hat.com, hpa@...or.com, torvalds@...ux-foundation.org,
	bp@...e.de
Subject: [tip:x86/asm] x86/entry/64/compat: Document sysenter_fix_flags'
 s reason for existence

Commit-ID:  374a3a3916a70fc6236bc2b8f8ac02548a128a54
Gitweb:     http://git.kernel.org/tip/374a3a3916a70fc6236bc2b8f8ac02548a128a54
Author:     Borislav Petkov <bp@...en8.de>
AuthorDate: Fri, 9 Oct 2015 19:08:59 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Sun, 11 Oct 2015 11:06:40 +0200

x86/entry/64/compat: Document sysenter_fix_flags's reason for existence

The code under the label can normally be inline, without the
jumping back and forth but the latter is an optimization.

Document that.

Signed-off-by: Borislav Petkov <bp@...e.de>
Acked-by: Andy Lutomirski <luto@...nel.org>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/20151009170859.GA24266@pd.tnic
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/entry/entry_64_compat.S | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index cf9641c..92b0b27 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -102,6 +102,12 @@ ENTRY(entry_SYSENTER_compat)
 	 * NT was set instead of doing an unconditional popfq.
 	 * This needs to happen before enabling interrupts so that
 	 * we don't get preempted with NT set.
+	 *
+	 * NB.: sysenter_fix_flags is a label with the code under it moved
+	 * out-of-line as an optimization: NT is unlikely to be set in the
+	 * majority of the cases and instead of polluting the I$ unnecessarily,
+	 * we're keeping that code behind a branch which will predict as
+	 * not-taken and therefore its instructions won't be fetched.
 	 */
 	testl	$X86_EFLAGS_NT, EFLAGS(%rsp)
 	jnz	sysenter_fix_flags
--
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