powerpc/64: Drop reservation-clearing ldarx in context switch

There is no need to explicitly break the reservation in _switch,
because we are guaranteed that the context switch path will include a
larx/stcx.

Comment the guarantee and remove the reservation clear from _switch.

This is worth 1-2% in context switch performance.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/entry_64.S

@@ -521,15 +521,10 @@ _GLOBAL(_switch)
 #endif /* CONFIG_SMP */
 
 	/*
-	 * If we optimise away the clear of the reservation in system
-	 * calls because we know the CPU tracks the address of the
-	 * reservation, then we need to clear it here to cover the
-	 * case that the kernel context switch path has no larx
-	 * instructions.
+	 * The kernel context switch path must contain a spin_lock,
+	 * which contains larx/stcx, which will clear any reservation
+	 * of the task being switched.
 	 */
-BEGIN_FTR_SECTION
-	ldarx	r6,0,r1
-END_FTR_SECTION_IFSET(CPU_FTR_STCX_CHECKS_ADDRESS)
 
 BEGIN_FTR_SECTION
 /*