x86/entry: Call enter_from_user_mode() with IRQs off

Now that slow-path syscalls always enter C before enabling
interrupts, it's straightforward to call enter_from_user_mode() before
enabling interrupts rather than doing it as part of entry tracing.

With this change, we should finally be able to retire exception_enter().

This will also enable optimizations based on knowing that we never
change context tracking state with interrupts on.

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: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/bc376ecf87921a495e874ff98139b1ca2f5c5dd7.1457558566.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 11 insertions, 22 deletions

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index d69d1b6e6c31..e79d93d44ecd 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -45,6 +45,8 @@ __visible void enter_from_user_mode(void)
         CT_WARN_ON(ct_state() != CONTEXT_USER);
         user_exit();
 }
+#else
+static inline void enter_from_user_mode(void) {}
 #endif
 
 static void do_audit_syscall_entry(struct pt_regs *regs, u32 arch)
@@ -85,17 +87,6 @@ unsigned long syscall_trace_enter_phase1(struct pt_regs *regs, u32 arch)
 
         work = ACCESS_ONCE(ti->flags) & _TIF_WORK_SYSCALL_ENTRY;
 
-#ifdef CONFIG_CONTEXT_TRACKING
-        /*
-         * If TIF_NOHZ is set, we are required to call user_exit() before
-         * doing anything that could touch RCU.
-         */
-        if (work & _TIF_NOHZ) {
-                enter_from_user_mode();
-                work &= ~_TIF_NOHZ;
-        }
-#endif
-
 #ifdef CONFIG_SECCOMP
         /*
          * Do seccomp first -- it should minimize exposure of other
@@ -344,6 +335,7 @@ __visible void do_syscall_64(struct pt_regs *regs)
         struct thread_info *ti = pt_regs_to_thread_info(regs);
         unsigned long nr = regs->orig_ax;
 
+        enter_from_user_mode();
         local_irq_enable();
 
         if (READ_ONCE(ti->flags) & _TIF_WORK_SYSCALL_ENTRY)
@@ -366,9 +358,9 @@ __visible void do_syscall_64(struct pt_regs *regs)
 
 #if defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION)
 /*
- * Does a 32-bit syscall.  Called with IRQs on and does all entry and
- * exit work and returns with IRQs off.  This function is extremely hot
- * in workloads that use it, and it's usually called from
+ * Does a 32-bit syscall.  Called with IRQs on in CONTEXT_KERNEL.  Does
+ * all entry and exit work and returns with IRQs off.  This function is
+ * extremely hot in workloads that use it, and it's usually called from
  * do_fast_syscall_32, so forcibly inline it to improve performance.
  */
 static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs)
@@ -409,6 +401,7 @@ static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs)
 /* Handles int $0x80 */
 __visible void do_int80_syscall_32(struct pt_regs *regs)
 {
+        enter_from_user_mode();
         local_irq_enable();
         do_syscall_32_irqs_on(regs);
 }
@@ -431,12 +424,11 @@ __visible long do_fast_syscall_32(struct pt_regs *regs)
          */
         regs->ip = landing_pad;
 
-        /*
-         * Fetch EBP from where the vDSO stashed it.
-         *
-         * WARNING: We are in CONTEXT_USER and RCU isn't paying attention!
-         */
+        enter_from_user_mode();
+
         local_irq_enable();
+
+        /* Fetch EBP from where the vDSO stashed it. */
         if (
 #ifdef CONFIG_X86_64
                 /*
@@ -454,9 +446,6 @@ __visible long do_fast_syscall_32(struct pt_regs *regs)
                 /* User code screwed up. */
                 local_irq_disable();
                 regs->ax = -EFAULT;
-#ifdef CONFIG_CONTEXT_TRACKING
-                enter_from_user_mode();
-#endif
                 prepare_exit_to_usermode(regs);
                 return 0;       /* Keep it simple: use IRET. */
         }