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>

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index e13027247c90b7ad5d4b19919f7c118a4a19b398..8daa8127f578f161fee425a4a2db08f528d0602a 100644 (file)
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -294,13 +294,29 @@ 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   $0                      /* pt_regs->ip = 0 (placeholder) */
        pushl   %eax                    /* pt_regs->orig_ax */
        SAVE_ALL pt_regs_ax=$-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