[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 e9f35c6..687ed35 100644
--- a/arch/x86_64/kernel/process.c
+++ b/arch/x86_64/kernel/process.c
@@ -487,33 +487,6 @@
}
/*
- * 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 @@
}
}
- disable_tsc(prev_p, next_p);
-
return prev_p;
}