linux-user: constrain execution on a single core

QEMU doesn't behave well when a multi-threaded binary is emulated and
the various threads are scheduled on different cores by Linux. The
problem manifests as a deadlock with the all the threads blocking on
futex().

This commit uses a workaround which is to select a random core at the
beginning of the initial thread and set the CPU affinity to it. Since
the vast majority of binaries don't change the CPU affinity themselves
and on Linux the affinity is inherited by child threads, this persists
across all threads of a multi-threaded application.

Signed-off-by: Petros Angelatos <[email protected]>

linux-user/main.c

@@ -641,6 +641,23 @@ int main(int argc, char **argv, char **envp)
 
     module_call_init(MODULE_INIT_TRACE);
     qemu_init_cpu_list();
+
+    cpu_set_t cpuset;
+    long ncpus;
+    int sched_cpu;
+
+    srand(time(NULL) + getpid());
+
+    ncpus = sysconf(_SC_NPROCESSORS_ONLN);
+    sched_cpu = rand() % ncpus;
+
+    CPU_ZERO(&cpuset);
+    CPU_SET(sched_cpu, &cpuset);
+
+    if (sched_setaffinity(getpid(), sizeof(cpuset), &cpuset)) {
+        fprintf(stderr, "Warning: Unable to set CPU affinity\n");
+    }
+
     module_call_init(MODULE_INIT_QOM);
 
     envlist = envlist_create();