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From: Andy Lutomirski <luto@kernel.org>
To: x86@kernel.org
Cc: linux-kernel@vger.kernel.org, Borislav Petkov <bp@alien8.de>,
	Oleg Nesterov <oleg@redhat.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Brian Gerst <brgerst@gmail.com>,
	Andy Lutomirski <luto@kernel.org>
Subject: [PATCH v2 02/12] x86/entry/compat: In SYSENTER, sink AC clearing below the existing FLAGS test
Date: Wed,  9 Mar 2016 19:00:25 -0800	[thread overview]
Message-ID: <90d6db2189f9add83bc7bddd75a0c19ebbd676b2.1457578375.git.luto@kernel.org> (raw)
In-Reply-To: <cover.1457578375.git.luto@kernel.org>
In-Reply-To: <cover.1457578375.git.luto@kernel.org>

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@kernel.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 89bcb4979e7a..8d3728131809 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:
 
-- 
2.5.0

  parent reply	other threads:[~2016-03-10  3:00 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-03-10  3:00 [PATCH v2 00/12] x86: Various SYSENTER/SYSEXIT/#DB fixes and cleanups Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 01/12] selftests/x86: In syscall_nt, test NT|TF as well Andy Lutomirski
2016-03-10 11:00   ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2016-03-10  3:00 ` Andy Lutomirski [this message]
2016-03-10 11:01   ` [tip:x86/asm] x86/entry/compat: In SYSENTER, sink AC clearing below the existing FLAGS test tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 03/12] x86/entry/32: Filter NT and speed up AC filtering in SYSENTER Andy Lutomirski
2016-03-10 11:01   ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 04/12] x86/entry/32: Restore FLAGS on SYSEXIT Andy Lutomirski
2016-03-10 11:01   ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 05/12] x86/traps: Clear TIF_BLOCKSTEP on all debug exceptions Andy Lutomirski
2016-03-10 11:02   ` [tip:x86/asm] x86/entry/traps: " tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 06/12] x86/traps: Clear DR6 early in do_debug and improve the comment Andy Lutomirski
2016-03-10 11:02   ` [tip:x86/asm] x86/entry/traps: Clear DR6 early in do_debug() " tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 07/12] x86/entry: Vastly simplify SYSENTER TF handling Andy Lutomirski
2016-03-10 11:03   ` [tip:x86/asm] x86/entry: Vastly simplify SYSENTER TF (single-step) handling tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 08/12] x86/entry: Only allocate space for SYSENTER_stack if needed Andy Lutomirski
2016-03-10 11:03   ` [tip:x86/asm] x86/entry: Only allocate space for tss_struct::SYSENTER_stack " tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 09/12] x86/entry/32: Simplify and fix up the SYSENTER stack #DB/NMI fixup Andy Lutomirski
2016-03-10 11:03   ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 10/12] x86/entry/32: Add and check a stack canary for the SYSENTER stack Andy Lutomirski
2016-03-10 11:04   ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 11/12] x86/entry: Remove TIF_SINGLESTEP entry work Andy Lutomirski
2016-03-10 11:04   ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 12/12] x86/entry: Improve system call entry comments Andy Lutomirski
2016-03-10 11:05   ` [tip:x86/asm] " tip-bot for Andy Lutomirski

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