make seccomp zerocost in schedule

This follows a suggestion from Chuck Ebbert on how to make seccomp
absolutely zerocost in schedule too.  The only remaining footprint of
seccomp is in terms of the bzImage size that becomes a few bytes (perhaps
even a few kbytes) larger, measure it if you care in the embedded.

Signed-off-by: Andrea Arcangeli <andrea@cpushare.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 39 insertions, 34 deletions

diff --git a/arch/i386/kernel/process.c b/arch/i386/kernel/process.c
index 06dfa65ad180..6c49acb96982 100644
--- a/arch/i386/kernel/process.c
+++ b/arch/i386/kernel/process.c
@@ -538,8 +538,31 @@ int dump_task_regs(struct task_struct *tsk, elf_gregset_t *regs)
         return 1;
 }
 
-static noinline void __switch_to_xtra(struct task_struct *next_p,
-                                    struct tss_struct *tss)
+#ifdef CONFIG_SECCOMP
+void hard_disable_TSC(void)
+{
+        write_cr4(read_cr4() | X86_CR4_TSD);
+}
+void disable_TSC(void)
+{
+        preempt_disable();
+        if (!test_and_set_thread_flag(TIF_NOTSC))
+                /*
+                 * Must flip the CPU state synchronously with
+                 * TIF_NOTSC in the current running context.
+                 */
+                hard_disable_TSC();
+        preempt_enable();
+}
+void hard_enable_TSC(void)
+{
+        write_cr4(read_cr4() & ~X86_CR4_TSD);
+}
+#endif /* CONFIG_SECCOMP */
+
+static noinline void
+__switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
+                 struct tss_struct *tss)
 {
         struct thread_struct *next;
 
@@ -555,6 +578,17 @@ static noinline void __switch_to_xtra(struct task_struct *next_p,
                 set_debugreg(next->debugreg[7], 7);
         }
 
+#ifdef CONFIG_SECCOMP
+        if (test_tsk_thread_flag(prev_p, TIF_NOTSC) ^
+            test_tsk_thread_flag(next_p, TIF_NOTSC)) {
+                /* prev and next are different */
+                if (test_tsk_thread_flag(next_p, TIF_NOTSC))
+                        hard_disable_TSC();
+                else
+                        hard_enable_TSC();
+        }
+#endif
+
         if (!test_tsk_thread_flag(next_p, TIF_IO_BITMAP)) {
                 /*
                  * Disable the bitmap via an invalid offset. We still cache
@@ -586,33 +620,6 @@ static noinline void __switch_to_xtra(struct task_struct *next_p,
 }
 
 /*
- * This function selects if the context switch from prev to next
- * has to tweak the TSC disable bit in the cr4.
- */
-static inline void disable_tsc(struct task_struct *prev_p,
-                               struct task_struct *next_p)
-{
-        struct thread_info *prev, *next;
-
-        /*
-         * gcc should eliminate the ->thread_info dereference if
-         * has_secure_computing returns 0 at compile time (SECCOMP=n).
-         */
-        prev = task_thread_info(prev_p);
-        next = task_thread_info(next_p);
-
-        if (has_secure_computing(prev) || has_secure_computing(next)) {
-                /* slow path here */
-                if (has_secure_computing(prev) &&
-                    !has_secure_computing(next)) {
-                        write_cr4(read_cr4() & ~X86_CR4_TSD);
-                } else if (!has_secure_computing(prev) &&
-                           has_secure_computing(next))
-                        write_cr4(read_cr4() | X86_CR4_TSD);
-        }
-}
-
-/*
  *      switch_to(x,yn) should switch tasks from x to y.
  *
  * We fsave/fwait so that an exception goes off at the right time
@@ -689,11 +696,9 @@ struct task_struct fastcall * __switch_to(struct task_struct *prev_p, struct tas
         /*
          * Now maybe handle debug registers and/or IO bitmaps
          */
-        if (unlikely((task_thread_info(next_p)->flags & _TIF_WORK_CTXSW)
-            || test_tsk_thread_flag(prev_p, TIF_IO_BITMAP)))
-                __switch_to_xtra(next_p, tss);
-
-        disable_tsc(prev_p, next_p);
+        if (unlikely(task_thread_info(prev_p)->flags & _TIF_WORK_CTXSW_PREV ||
+                     task_thread_info(next_p)->flags & _TIF_WORK_CTXSW_NEXT))
+                __switch_to_xtra(prev_p, next_p, tss);
 
         /*
          * Leave lazy mode, flushing any hypercalls made here.