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

From: tip-bot for Andy Lutomirski
Date: Thu Mar 10 2016 - 06:02:54 EST


Commit-ID: 67f590e8d4d718d9bd377b39223f7f69678d6a10
Gitweb: http://git.kernel.org/tip/67f590e8d4d718d9bd377b39223f7f69678d6a10
Author: Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Wed, 9 Mar 2016 19:00:26 -0800
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
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@xxxxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/07daef3d44bd1ed62a2c866e143e8df64edb40ee.1457578375.git.luto@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
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