[PATCH] x86-64: Remove disable_tsc code in context switch

It only offers extremly dubious security advantages and
is not worth the overhead in this critical path.

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/arch/x86_64/kernel/process.c b/arch/x86_64/kernel/process.c
index e9f35c60f8c6f1bf47443143653beccfbcac3fb7..687ed357e685e344cdd32e6953e8628f1b00256e 100644 (file)
--- a/arch/x86_64/kernel/process.c
+++ b/arch/x86_64/kernel/process.c
@@ -486,33 +486,6 @@ out:
        return err;
 }
 
-/*
- * 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 = prev_p->thread_info;
-       next = next_p->thread_info;
-
-       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);
-       }
-}
-
 /*
  * This special macro can be used to load a debugging register
  */
@@ -631,8 +604,6 @@ struct task_struct *__switch_to(struct task_struct *prev_p, struct task_struct *
                }
        }
 
-       disable_tsc(prev_p, next_p);
-
        return prev_p;
 }