kvm: Conditionally apply workaround for KVM slot handling bug

Only apply the workaround for broken slot joining in KVM when the
capability was not found that signals the corresponding fix existence.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

kvm-all.c

@@ -57,6 +57,7 @@ struct KVMState
     int fd;
     int vmfd;
     int coalesced_mmio;
+    int broken_set_mem_region;
 #ifdef KVM_CAP_SET_GUEST_DEBUG
     struct kvm_sw_breakpoint_head kvm_sw_breakpoints;
 #endif
@@ -406,6 +407,14 @@ int kvm_init(int smp_cpus)
     s->coalesced_mmio = 0;
 #endif
 
+    s->broken_set_mem_region = 1;
+#ifdef KVM_CAP_JOIN_MEMORY_REGIONS_WORKS
+    ret = kvm_ioctl(s, KVM_CHECK_EXTENSION, KVM_CAP_JOIN_MEMORY_REGIONS_WORKS);
+    if (ret > 0) {
+        s->broken_set_mem_region = 0;
+    }
+#endif
+
     ret = kvm_arch_init(s, smp_cpus);
     if (ret < 0)
         goto err;
@@ -639,7 +648,8 @@ void kvm_set_phys_mem(target_phys_addr_t start_addr,
          * address as the first existing one. If not or if some overlapping
          * slot comes around later, we will fail (not seen in practice so far)
          * - and actually require a recent KVM version. */
-        if (old.start_addr == start_addr && old.memory_size < size &&
+        if (s->broken_set_mem_region &&
+            old.start_addr == start_addr && old.memory_size < size &&
             flags < IO_MEM_UNASSIGNED) {
             mem = kvm_alloc_slot(s);
             mem->memory_size = old.memory_size;