powerpc/64s/hash: Fix assert_slb_presence() use of the slbfee. instruction

The slbfee. instruction must have bit 24 of RB clear, failure to do
so can result in false negatives that result in incorrect assertions.

This is not obvious from the ISA v3.0B document, which only says:

    The hardware ignores the contents of RB 36:38 40:63 -- p.1032

This patch fixes the bug and also clears all other bits from PPC bit
36-63, which is good practice when dealing with reserved or ignored
bits.

Fixes: e15a4fea ("powerpc/64s/hash: Add some SLB debugging tests")
Cc: stable@vger.kernel.org # v4.20+
Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Tested-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/slb.c

@@ -69,6 +69,11 @@ static void assert_slb_presence(bool present, unsigned long ea)
 	if (!cpu_has_feature(CPU_FTR_ARCH_206))
 		return;
 
+	/*
+	 * slbfee. requires bit 24 (PPC bit 39) be clear in RB. Hardware
+	 * ignores all other bits from 0-27, so just clear them all.
+	 */
+	ea &= ~((1UL << 28) - 1);
 	asm volatile(__PPC_SLBFEE_DOT(%0, %1) : "=r"(tmp) : "r"(ea) : "cr0");
 
 	WARN_ON(present == (tmp == 0));