Re: [Qemu-devel] [PATCH] qemu will core dump with "-smp 254, sockets=2, cores=3, threads=2"

[lijun <junmuzi@gmail.com>]

Hi all,
     As qemu core dump cause by "sockets=2,cores=3,threads=2", so add 
this patch to check whether cores and threads is a power of 2.
     The following is the realization of apicid_from_topo_ids function 
in file target-i386/topology.h. It uses shift to get the values of 
pkg_id and core_id. nr_cores and nr_threads is related to this shift.
static inline apic_id_t apicid_from_topo_ids(unsigned nr_cores,
                                              unsigned nr_threads,
                                              unsigned pkg_id,
                                              unsigned core_id,
                                              unsigned smt_id)
{
     return (pkg_id  << apicid_pkg_offset(nr_cores, nr_threads)) |
            (core_id << apicid_core_offset(nr_cores, nr_threads)) |
            smt_id;
}
----
So should add a check for smp_cores and smp_threads in smp_parse 
function in file vl.c. Check whether smp_cores and smp_threads is a 
power of 2, so nr_cores and nr_threads is a power of 2.  When return 
from apicid_from_topo_ids function, apic_id and id could get the correct 
values(apic_id is in file "hw/i386/acpi-build.c" and id is in file 
"hw/acpi/piix4.c") .

Without this check for smp_cores and smp_threads, specify "-smp 
160,sockets=2,cores=3,threads=2", qemu will core dump too.


--- a/vl.c    2013-12-14 23:46:58.991076467 +0800
+++ b/vl.c    2013-12-15 00:40:31.653800907 +0800
@@ -1384,6 +1384,19 @@
      },
  };

+/**
+ * This function will return whether @num is power of 2.
+ *
+ * Returns: 1 indicate @num is power of 2, 0 indicate @num is not.
+ */
+static int is_2_power(int num)
+{
+    if (num < 0 || num > 256)
+        return 1;
+
+    return !(num & (num - 1));
+}
+
  static void smp_parse(QemuOpts *opts)
  {
      if (opts) {
@@ -1418,6 +1431,12 @@

      }

+    /* check whether smp_cores and smp_threads is a power of 2 */
+    if (!is_2_power(smp_cores) || !is_2_power(smp_threads)) {
+        smp_cores = 1;
+        smp_threads = 1;
+    }
+
      if (max_cpus == 0) {
          max_cpus = smp_cpus;
      }


Best Regards,
Jun Li