x86/entry: Remove unnecessary IRQ twiddling in fast 32-bit syscalls

This is slightly messy, but it eliminates an unnecessary cli;sti
pair.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
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: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/22f34b1096694a37326f36c53407b8dd90f37948.1444091585.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/common.c

@@ -323,10 +323,10 @@ __visible void syscall_return_slowpath(struct pt_regs *regs)
 
 #if defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION)
 /*
- * Does a 32-bit syscall.  Called with IRQs off and does all entry and
- * exit work.
+ * Does a 32-bit syscall.  Called with IRQs on and does all entry and
+ * exit work and returns with IRQs off.
  */
-__visible void do_int80_syscall_32(struct pt_regs *regs)
+static void do_syscall_32_irqs_on(struct pt_regs *regs)
 {
 	struct thread_info *ti = pt_regs_to_thread_info(regs);
 	unsigned int nr = (unsigned int)regs->orig_ax;
@@ -335,8 +335,6 @@ __visible void do_int80_syscall_32(struct pt_regs *regs)
 	ti->status |= TS_COMPAT;
 #endif
 
-	local_irq_enable();
-
 	if (READ_ONCE(ti->flags) & _TIF_WORK_SYSCALL_ENTRY) {
 		/*
 		 * Subtlety here: if ptrace pokes something larger than
@@ -363,6 +361,13 @@ __visible void do_int80_syscall_32(struct pt_regs *regs)
 	syscall_return_slowpath(regs);
 }
 
+/* Handles int $0x80 */
+__visible void do_int80_syscall_32(struct pt_regs *regs)
+{
+	local_irq_enable();
+	do_syscall_32_irqs_on(regs);
+}
+
 /* Returns 0 to return using IRET or 1 to return using SYSEXIT/SYSRETL. */
 __visible long do_fast_syscall_32(struct pt_regs *regs)
 {
@@ -398,10 +403,9 @@ __visible long do_fast_syscall_32(struct pt_regs *regs)
 		prepare_exit_to_usermode(regs);
 		return 0;	/* Keep it simple: use IRET. */
 	}
-	local_irq_disable();
 
 	/* Now this is just like a normal syscall. */
-	do_int80_syscall_32(regs);
+	do_syscall_32_irqs_on(regs);
 
 #ifdef CONFIG_X86_64
 	/*