ioapic: keep RO bits for IOAPIC entry

Currently IOAPIC RO bits can be written. To be better aligned with
hardware, we should let them read-only.
Reviewed-by: NRadim Krčmář <rkrcmar@redhat.com>
Signed-off-by: NPeter Xu <peterx@redhat.com>
Message-Id: <1462875682-1349-2-git-send-email-peterx@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/intc/ioapic.c

@@ -281,6 +281,7 @@ ioapic_mem_write(void *opaque, hwaddr addr, uint64_t val,
         default:
             index = (s->ioregsel - IOAPIC_REG_REDTBL_BASE) >> 1;
             if (index >= 0 && index < IOAPIC_NUM_PINS) {
+                uint64_t ro_bits = s->ioredtbl[index] & IOAPIC_RO_BITS;
                 if (s->ioregsel & 1) {
                     s->ioredtbl[index] &= 0xffffffff;
                     s->ioredtbl[index] |= (uint64_t)val << 32;
@@ -288,6 +289,9 @@ ioapic_mem_write(void *opaque, hwaddr addr, uint64_t val,
                     s->ioredtbl[index] &= ~0xffffffffULL;
                     s->ioredtbl[index] |= val;
                 }
+                /* restore RO bits */
+                s->ioredtbl[index] &= IOAPIC_RW_BITS;
+                s->ioredtbl[index] |= ro_bits;
                 ioapic_service(s);
             }
         }

include/hw/i386/ioapic_internal.h

@@ -47,6 +47,11 @@
 #define IOAPIC_LVT_DEST_MODE            (1 << IOAPIC_LVT_DEST_MODE_SHIFT)
 #define IOAPIC_LVT_DELIV_MODE           (7 << IOAPIC_LVT_DELIV_MODE_SHIFT)
 
+/* Bits that are read-only for IOAPIC entry */
+#define IOAPIC_RO_BITS                  (IOAPIC_LVT_REMOTE_IRR | \
+                                         IOAPIC_LVT_DELIV_STATUS)
+#define IOAPIC_RW_BITS                  (~(uint64_t)IOAPIC_RO_BITS)
+
 #define IOAPIC_TRIGGER_EDGE             0
 #define IOAPIC_TRIGGER_LEVEL            1