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

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Date:	Fri, 9 Oct 2015 19:08:59 +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: [PATCH] x86/entry/64/compat: Document sysenter_fix_flags's reason
 for existence

From: Borislav Petkov <bp@...e.de>
Date: Fri, 9 Oct 2015 19:04:09 +0200
Subject: [PATCH] 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>
---
 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 cf9641cd4796..92b0b27b43c6 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
-- 
2.3.5

-- 
Regards/Gruss,
    Boris.

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