powerpc/mm/hash: hard disable irq in the SLB insert path

When inserting SLB entries for EA above 512TB, we need to hard disable irq.
This will make sure we don't take a PMU interrupt that can possibly touch
user space address via a stack dump. To prevent this, we need to hard disable
the interrupt.

Also add a comment explaining why we don't need context synchronizing isync
with slbmte.

Fixes: f384796c ("powerpc/mm: Add support for handling > 512TB address in SLB miss")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/slb.c

@@ -352,6 +352,14 @@ static void insert_slb_entry(unsigned long vsid, unsigned long ea,
 	/*
 	 * We are irq disabled, hence should be safe to access PACA.
 	 */
+	VM_WARN_ON(!irqs_disabled());
+
+	/*
+	 * We can't take a PMU exception in the following code, so hard
+	 * disable interrupts.
+	 */
+	hard_irq_disable();
+
 	index = get_paca()->stab_rr;
 
 	/*
@@ -369,6 +377,11 @@ static void insert_slb_entry(unsigned long vsid, unsigned long ea,
 		    ((unsigned long) ssize << SLB_VSID_SSIZE_SHIFT);
 	esid_data = mk_esid_data(ea, ssize, index);
 
+	/*
+	 * No need for an isync before or after this slbmte. The exception
+	 * we enter with and the rfid we exit with are context synchronizing.
+	 * Also we only handle user segments here.
+	 */
 	asm volatile("slbmte %0, %1" : : "r" (vsid_data), "r" (esid_data)
 		     : "memory");