cpus-common: ensure auto-assigned cpu_indexes don't clash

Basing the cpu_index on the number of currently allocated vCPUs fails
when vCPUs aren't removed in a LIFO manner. This is especially true
when we are allocating a cpu_index for each guest thread in
linux-user where there is no ordering constraint on their allocation
and de-allocation.

[I've dropped the assert which is there to guard against out-of-order
removal as this should probably be caught higher up the stack. Maybe
we could just ifdef CONFIG_SOFTTMU it?]

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Igor Mammedow <imammedo@redhat.com>
Cc: Nikolay Igotti <igotti@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20200520140541.30256-13-alex.bennee@linaro.org>

cpus-common.c

@@ -61,13 +61,15 @@ static bool cpu_index_auto_assigned;
 static int cpu_get_free_index(void)
 {
     CPUState *some_cpu;
-    int cpu_index = 0;
+    int max_cpu_index = 0;
 
     cpu_index_auto_assigned = true;
     CPU_FOREACH(some_cpu) {
-        cpu_index++;
+        if (some_cpu->cpu_index >= max_cpu_index) {
+            max_cpu_index = some_cpu->cpu_index + 1;
+        }
     }
-    return cpu_index;
+    return max_cpu_index;
 }
 
 void cpu_list_add(CPUState *cpu)
@@ -90,8 +92,6 @@ void cpu_list_remove(CPUState *cpu)
         return;
     }
 
-    assert(!(cpu_index_auto_assigned && cpu != QTAILQ_LAST(&cpus)));
-
     QTAILQ_REMOVE_RCU(&cpus, cpu, node);
     cpu->cpu_index = UNASSIGNED_CPU_INDEX;
 }