linux-kernel - [tip:x86/asm] x86/entry/compat: In SYSENTER, sink AC clearing below the existing FLAGS test

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Date:	Thu, 10 Mar 2016 03:01:06 -0800
From:	tip-bot for Andy Lutomirski <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	luto@...capital.net, brgerst@...il.com, oleg@...hat.com,
	peterz@...radead.org, tglx@...utronix.de, bp@...en8.de,
	luto@...nel.org, andrew.cooper3@...rix.com,
	torvalds@...ux-foundation.org, hpa@...or.com, mingo@...nel.org,
	dvlasenk@...hat.com, linux-kernel@...r.kernel.org
Subject: [tip:x86/asm] x86/entry/compat: In SYSENTER, sink AC clearing below
 the existing FLAGS test

Commit-ID:  e786041153df6343169373177248abfab5c5ac1b
Gitweb:     http://git.kernel.org/tip/e786041153df6343169373177248abfab5c5ac1b
Author:     Andy Lutomirski <luto@...nel.org>
AuthorDate: Wed, 9 Mar 2016 19:00:25 -0800
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Thu, 10 Mar 2016 09:48:12 +0100

x86/entry/compat: In SYSENTER, sink AC clearing below the existing FLAGS test

CLAC is slow, and the SYSENTER code already has an unlikely path
that runs if unusual flags are set.  Drop the CLAC and instead rely
on the unlikely path to clear AC.

This seems to save ~24 cycles on my Skylake laptop.  (Hey, Intel,
make this faster please!)

Signed-off-by: Andy Lutomirski <luto@...nel.org>
Cc: Andrew Cooper <andrew.cooper3@...rix.com>
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: Oleg Nesterov <oleg@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/90d6db2189f9add83bc7bddd75a0c19ebbd676b2.1457578375.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/entry/entry_64_compat.S | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 3c990ee..da5c194 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -66,8 +66,6 @@ ENTRY(entry_SYSENTER_compat)
 	 */
 	pushfq				/* pt_regs->flags (except IF = 0) */
 	orl	$X86_EFLAGS_IF, (%rsp)	/* Fix saved flags */
-	ASM_CLAC			/* Clear AC after saving FLAGS */
-
 	pushq	$__USER32_CS		/* pt_regs->cs */
 	xorq    %r8,%r8
 	pushq	%r8			/* pt_regs->ip = 0 (placeholder) */
@@ -90,9 +88,9 @@ ENTRY(entry_SYSENTER_compat)
 	cld
 
 	/*
-	 * Sysenter doesn't filter flags, so we need to clear NT
+	 * SYSENTER doesn't filter flags, so we need to clear NT and AC
 	 * ourselves.  To save a few cycles, we can check whether
-	 * NT was set instead of doing an unconditional popfq.
+	 * either 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.
 	 *
@@ -102,7 +100,7 @@ ENTRY(entry_SYSENTER_compat)
 	 * 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)
+	testl	$X86_EFLAGS_NT|X86_EFLAGS_AC, EFLAGS(%rsp)
 	jnz	.Lsysenter_fix_flags
 .Lsysenter_flags_fixed: