x86: debugctlmsr context switch

This adds low-level support for a per-thread value of MSR_IA32_DEBUGCTLMSR.
The per-thread value is switched in when TIF_DEBUGCTLMSR is set.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/process_32.c

@@ -602,10 +602,14 @@ static noinline void
 __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
 		 struct tss_struct *tss)
 {
-	struct thread_struct *next;
+	struct thread_struct *prev, *next;
 
+	prev = &prev_p->thread;
 	next = &next_p->thread;
 
+	if (next->debugctlmsr != prev->debugctlmsr)
+		wrmsr(MSR_IA32_DEBUGCTLMSR, next->debugctlmsr, 0);
+
 	if (test_tsk_thread_flag(next_p, TIF_DEBUG)) {
 		set_debugreg(next->debugreg[0], 0);
 		set_debugreg(next->debugreg[1], 1);

arch/x86/kernel/process_64.c

@@ -555,6 +555,9 @@ static inline void __switch_to_xtra(struct task_struct *prev_p,
 	prev = &prev_p->thread,
 	next = &next_p->thread;
 
+	if (next->debugctlmsr != prev->debugctlmsr)
+		wrmsrl(MSR_IA32_DEBUGCTLMSR, next->debugctlmsr);
+
 	if (test_tsk_thread_flag(next_p, TIF_DEBUG)) {
 		loaddebug(next, 0);
 		loaddebug(next, 1);