[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>

1 files changed, 0 insertions, 29 deletions

diff --git a/arch/x86_64/kernel/process.c b/arch/x86_64/kernel/process.c
index e9f35c60f8c6..687ed357e685 100644
--- a/arch/x86_64/kernel/process.c
+++ b/arch/x86_64/kernel/process.c
@@ -487,33 +487,6 @@ out:
 }
 
 /*
- * 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
  */
 #define loaddebug(thread,r) set_debug(thread->debugreg ## r, r)
@@ -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;
 }