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: <20241219034035.1826173-5-npiggin@gmail.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
diff --git a/target/ppc/mmu-radix64.c b/target/ppc/mmu-radix64.c
index 1d3d9e1..461eda4 100644
--- a/target/ppc/mmu-radix64.c
+++ b/target/ppc/mmu-radix64.c
@@ -571,6 +571,20 @@
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;
