powerpc/pseries: lift RTAS limit for hash

With the previous patch to switch to 64-bit mode after returning from
RTAS and before doing any memory accesses, the RMA limit need not be
clamped to 1GB to avoid RTAS bugs.

Keep the 1GB limit for older firmware (although this is more of a kernel
concern than RTAS), and remove it starting with POWER9.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/hash_utils_64.c

@@ -1853,11 +1853,13 @@ void hash__setup_initial_memory_limit(phys_addr_t first_memblock_base,
 	 * non-virtualized 64-bit hash MMU systems don't have a limitation
 	 * on real mode access.
 	 *
-	 * We also clamp it to 1G to avoid some funky things
-	 * such as RTAS bugs etc...
+	 * For guests on platforms before POWER9, we clamp the it limit to 1G
+	 * to avoid some funky things such as RTAS bugs etc...
 	 */
 	if (!early_cpu_has_feature(CPU_FTR_HVMODE)) {
-		ppc64_rma_size = min_t(u64, first_memblock_size, 0x40000000);
+		ppc64_rma_size = first_memblock_size;
+		if (!early_cpu_has_feature(CPU_FTR_ARCH_300))
+			ppc64_rma_size = min_t(u64, ppc64_rma_size, 0x40000000);
 
 		/* Finally limit subsequent allocations */
 		memblock_set_current_limit(ppc64_rma_size);