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Commit 083044e6 authored by Andi Kleen's avatar Andi Kleen Committed by Linus Torvalds
Browse files

[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: default avatarAndi Kleen <ak@suse.de>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent fe5d5f07

arch/x86_64/kernel/process.c

+0 −29
Original line number Diff line number Diff line
@@ -486,33 +486,6 @@ int copy_thread(int nr, unsigned long clone_flags, unsigned long rsp, 
	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;

}