target/ppc: Avoid warning message for zero process table entries

A translation that encounters a process table entry that is zero is
something that Linux does to cause certain kernel NULL pointer
dereferences to fault. It is not itself a programming error, so avoid
the guest error log.

Message-ID: <[email protected]>
Signed-off-by: Nicholas Piggin <[email protected]>

Author: npiggin

target/ppc/mmu-radix64.c

@@ -571,6 +571,20 @@ static int ppc_radix64_process_scoped_xlate(PowerPCCPU *cpu,
         prtbe0 = ldq_phys(cs->as, h_raddr);
     }
 
+    /*
+     * Some Linux uses a zero process table entry in PID!=0 for kernel context
+     * without userspace in order to fault on NULL dereference, because using
+     * PIDR=0 for the kernel causes the Q0 page table to be used to translate
+     * Q3 as well. Check for that case here to avoid the invalid configuration
+     * message.
+     */
+    if (unlikely(!prtbe0)) {
+        if (guest_visible) {
+            ppc_radix64_raise_si(cpu, access_type, eaddr, DSISR_R_BADCONFIG);
+        }
+        return 1;
+    }
+
     /* Walk Radix Tree from Process Table Entry to Convert EA to RA */
     *g_page_size = PRTBE_R_GET_RTS(prtbe0);
     base_addr = prtbe0 & PRTBE_R_RPDB;