powerpc/64: Optimise set/clear of CTRL[RUN] (runlatch)

On modern CPUs the CTRL register is read-only except bit 63 which is
the run latch control. This means it can be updated with a mtspr
rather than mfspr/mtspr.

To accomodate older CPUs (Cell at least), where there are other bits
in the register, we still do a read/modify/write on pre 2.06 CPUs.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Update change log to mention 2.06 workaround]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/process.c

@@ -1979,11 +1979,25 @@ void show_stack(struct task_struct *tsk, unsigned long *stack)
 void notrace __ppc64_runlatch_on(void)
 {
 	struct thread_info *ti = current_thread_info();
+
+	if (cpu_has_feature(CPU_FTR_ARCH_206)) {
+		/*
+		 * Least significant bit (RUN) is the only writable bit of
+		 * the CTRL register, so we can avoid mfspr. 2.06 is not the
+		 * earliest ISA where this is the case, but it's convenient.
+		 */
+		mtspr(SPRN_CTRLT, CTRL_RUNLATCH);
+	} else {
 		unsigned long ctrl;
 
+		/*
+		 * Some architectures (e.g., Cell) have writable fields other
+		 * than RUN, so do the read-modify-write.
+		 */
 		ctrl = mfspr(SPRN_CTRLF);
 		ctrl |= CTRL_RUNLATCH;
 		mtspr(SPRN_CTRLT, ctrl);
+	}
 
 	ti->local_flags |= _TLF_RUNLATCH;
 }
@@ -1992,13 +2006,18 @@ void notrace __ppc64_runlatch_on(void)
 void notrace __ppc64_runlatch_off(void)
 {
 	struct thread_info *ti = current_thread_info();
-	unsigned long ctrl;
 
 	ti->local_flags &= ~_TLF_RUNLATCH;
 
+	if (cpu_has_feature(CPU_FTR_ARCH_206)) {
+		mtspr(SPRN_CTRLT, 0);
+	} else {
+		unsigned long ctrl;
+
 		ctrl = mfspr(SPRN_CTRLF);
 		ctrl &= ~CTRL_RUNLATCH;
 		mtspr(SPRN_CTRLT, ctrl);
+	}
 }
 #endif /* CONFIG_PPC64 */