exec: make iotlb RCU-friendly

After the previous patch, TLBs will be flushed on every change to
the memory mapping.  This patch augments that with synchronization
of the MemoryRegionSections referred to in the iotlb array.

With this change, it is guaranteed that iotlb_to_region will access
the correct memory map, even once the TLB will be accessed outside
the BQL.

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

cpu-exec.c

@@ -24,6 +24,8 @@
 #include "qemu/atomic.h"
 #include "sysemu/qtest.h"
 #include "qemu/timer.h"
+#include "exec/address-spaces.h"
+#include "exec/memory-internal.h"
 
 /* -icount align implementation. */
 
@@ -144,7 +146,9 @@ void cpu_resume_from_signal(CPUState *cpu, void *puc)
 
 void cpu_reload_memory_map(CPUState *cpu)
 {
-    /* The TLB is protected by the iothread lock.  */
+    /* The CPU and TLB are protected by the iothread lock.  */
+    AddressSpaceDispatch *d = cpu->as->dispatch;
+    cpu->memory_dispatch = d;
     tlb_flush(cpu, 1);
 }
 #endif