[PATCH v2 02/12] x86/entry/compat: In SYSENTER, sink AC clearing below the existing FLAGS test

[Andy Lutomirski]

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@xxxxxxxxxx>
---
 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