powerpc/64s: Leave interrupts hard enabled in context switch for radix

Commit 4387e9ff25 ("[POWERPC] Fix PMU + soft interrupt disable bug")
hard disabled interrupts over the low level context switch, because
the SLB management can't cope with a PMU interrupt accesing the stack
in that window.

Radix based kernel mapping does not use the SLB so it does not require
interrupts hard disabled here.

This is worth 1-2% in context switch performance on POWER9.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/entry_64.S

@@ -607,6 +607,14 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_1T_SEGMENT)
 	   top of the kernel stack. */
 	addi	r7,r7,THREAD_SIZE-SWITCH_FRAME_SIZE
 
+	/*
+	 * PMU interrupts in radix may come in here. They will use r1, not
+	 * PACAKSAVE, so this stack switch will not cause a problem. They
+	 * will store to the process stack, which may then be migrated to
+	 * another CPU. However the rq lock release on this CPU paired with
+	 * the rq lock acquire on the new CPU before the stack becomes
+	 * active on the new CPU, will order those stores.
+	 */
 	mr	r1,r8		/* start using new stack pointer */
 	std	r7,PACAKSAVE(r13)
 

arch/powerpc/kernel/process.c

@@ -1199,12 +1199,14 @@ struct task_struct *__switch_to(struct task_struct *prev,
 
 	__switch_to_tm(prev, new);
 
-	/*
-	 * We can't take a PMU exception inside _switch() since there is a
-	 * window where the kernel stack SLB and the kernel stack are out
-	 * of sync. Hard disable here.
-	 */
-	hard_irq_disable();
+	if (!radix_enabled()) {
+		/*
+		 * We can't take a PMU exception inside _switch() since there
+		 * is a window where the kernel stack SLB and the kernel stack
+		 * are out of sync. Hard disable here.
+		 */
+		hard_irq_disable();
+	}
 
 	/*
 	 * Call restore_sprs() before calling _switch(). If we move it after