trace: add sanity check

If trace backend is set to TRACE_NOP, trace_get_vcpu_event_count
returns 0, cause bitmap_new call abort.

The abort can be triggered as follows:

  $ ./configure --enable-trace-backend=nop --target-list=x86_64-softmmu
  $ gdb ./x86_64-softmmu/qemu-system-x86_64 -M q35,accel=kvm -m 1G
  (gdb) bt
  #0  0x00007ffff04e25f7 in raise () from /lib64/libc.so.6
  #1  0x00007ffff04e3ce8 in abort () from /lib64/libc.so.6
  #2  0x00005555559de905 in bitmap_new (nbits=<optimized out>)
      at /home/root/git/qemu2.git/include/qemu/bitmap.h:96
  qemu#3  cpu_common_initfn (obj=0x555556621d30) at qom/cpu.c:399
  qemu#4  0x0000555555a11869 in object_init_with_type (obj=0x555556621d30, ti=0x55555656bbb0) at qom/object.c:341
  qemu#5  0x0000555555a11869 in object_init_with_type (obj=0x555556621d30, ti=0x55555656bd30) at qom/object.c:341
  qemu#6  0x0000555555a11efc in object_initialize_with_type (data=data@entry=0x555556621d30, size=76560,
      type=type@entry=0x55555656bd30) at qom/object.c:376
  qemu#7  0x0000555555a12061 in object_new_with_type (type=0x55555656bd30) at qom/object.c:484
  qemu#8  0x0000555555a121c5 in object_new (typename=typename@entry=0x555556550340 "qemu64-x86_64-cpu")
      at qom/object.c:494
  qemu#9  0x00005555557f6e3d in pc_new_cpu (typename=typename@entry=0x555556550340 "qemu64-x86_64-cpu", apic_id=0,
      errp=errp@entry=0x5555565391b0 <error_fatal>) at /home/root/git/qemu2.git/hw/i386/pc.c:1101
  qemu#10 0x00005555557fa33e in pc_cpus_init (pcms=pcms@entry=0x5555565f9690)
      at /home/root/git/qemu2.git/hw/i386/pc.c:1184
  qemu#11 0x00005555557fe0f6 in pc_q35_init (machine=0x5555565f9690) at /home/root/git/qemu2.git/hw/i386/pc_q35.c:121
  qemu#12 0x000055555574fbad in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:4562

Signed-off-by: Anthony Xu <anthony.xu@intel.com>
Message-id: 1494369432-15418-1-git-send-email-anthony.xu@intel.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

qom/cpu.c

@@ -382,6 +382,7 @@ static void cpu_common_unrealizefn(DeviceState *dev, Error **errp)
 
 static void cpu_common_initfn(Object *obj)
 {
+    uint32_t count;
     CPUState *cpu = CPU(obj);
     CPUClass *cc = CPU_GET_CLASS(obj);
 
@@ -396,7 +397,10 @@ static void cpu_common_initfn(Object *obj)
     QTAILQ_INIT(&cpu->breakpoints);
     QTAILQ_INIT(&cpu->watchpoints);
 
-    cpu->trace_dstate = bitmap_new(trace_get_vcpu_event_count());
+    count = trace_get_vcpu_event_count();
+    if (count) {
+        cpu->trace_dstate = bitmap_new(count);
+    }
 
     cpu_exec_initfn(cpu);
 }