[tip:x86/asm] x86/entry/32: Filter NT and speed up AC filtering in SYSENTER

[tip-bot for Andy Lutomirski <tipbot@zytor.com>]

Commit-ID:  67f590e8d4d718d9bd377b39223f7f69678d6a10
Gitweb:     http://git.kernel.org/tip/67f590e8d4d718d9bd377b39223f7f69678d6a10
Author:     Andy Lutomirski <luto@kernel.org>
AuthorDate: Wed, 9 Mar 2016 19:00:26 -0800
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 10 Mar 2016 09:48:12 +0100

x86/entry/32: Filter NT and speed up AC filtering in SYSENTER

This makes the 32-bit code work just like the 64-bit code.  It should
speed up syscalls on 32-bit kernels on Skylake by something like 20
cycles (by analogy to the 64-bit compat case).

It also cleans up NT just like we do for the 64-bit case.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/07daef3d44bd1ed62a2c866e143e8df64edb40ee.1457578375.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/entry/entry_32.S | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index e130272..8daa812 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -294,7 +294,6 @@ sysenter_past_esp:
 	pushl	$__USER_DS		/* pt_regs->ss */
 	pushl	%ebp			/* pt_regs->sp (stashed in bp) */
 	pushfl				/* pt_regs->flags (except IF = 0) */
-	ASM_CLAC			/* Clear AC after saving FLAGS */
 	orl	$X86_EFLAGS_IF, (%esp)	/* Fix IF */
 	pushl	$__USER_CS		/* pt_regs->cs */
 	pushl	$0			/* pt_regs->ip = 0 (placeholder) */
@@ -302,6 +301,23 @@ sysenter_past_esp:
 	SAVE_ALL pt_regs_ax=$-ENOSYS	/* save rest */
 
 	/*
+	 * SYSENTER doesn't filter flags, so we need to clear NT and AC
+	 * ourselves.  To save a few cycles, we can check whether
+	 * 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.
+	 *
+	 * NB.: .Lsysenter_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|X86_EFLAGS_AC, PT_EFLAGS(%esp)
+	jnz	.Lsysenter_fix_flags
+.Lsysenter_flags_fixed:
+
+	/*
 	 * User mode is traced as though IRQs are on, and SYSENTER
 	 * turned them off.
 	 */
@@ -339,6 +355,11 @@ sysenter_past_esp:
 .popsection
 	_ASM_EXTABLE(1b, 2b)
 	PTGS_TO_GS_EX
+
+.Lsysenter_fix_flags:
+	pushl	$X86_EFLAGS_FIXED
+	popfl
+	jmp	.Lsysenter_flags_fixed
 ENDPROC(entry_SYSENTER_32)
 
 	# system call handler stub