Currently it is not forbidden to use "-object memory-backend-file,share=on"
and together with "-incoming". But after incoming migration is finished,
the memory-backend-file will be definitely written if share=on. So the
memory-backend-file can only be used once, but failed in the 2nd time
incoming.

Thus it gives a warning and the users can run the qemu if they really
want to do it.

Signed-off-by: Catherine Ho <catherine.hecx@gmail.com>
---
 backends/hostmem-file.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
index 37ac6445d2..ce03dc0a18 100644
--- a/backends/hostmem-file.c
+++ b/backends/hostmem-file.c
@@ -16,6 +16,7 @@
 #include "sysemu/hostmem.h"
 #include "sysemu/sysemu.h"
 #include "qom/object_interfaces.h"
+#include "migration/migration.h"
 
 /* hostmem-file.c */
 /**
@@ -79,6 +80,17 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
         }
     }
 
+    /*
+     * In ignore shared incoming migration, if share=on for host memory
+     * backend file, the ram might be modified after incoming process.
+     * The user should know this potential risk.
+     */
+    if (backend->share && migrate_ignore_shared()
+                       && runstate_check(RUN_STATE_INMIGRATE))
+        warn_report("NOTE: Please make sure the data on the shared memory "
+                    "backend file and the data from the incoming migration"
+                    " stream contains matching contents, otherwise...");
+
     backend->force_prealloc = mem_prealloc;
     name = host_memory_backend_get_name(backend);
     memory_region_init_ram_from_file(&backend->mr, OBJECT(backend),
-- 
2.17.1