Deprecate pmem=on with non-DAX capable backend file

It is not safe to pretend that emulated NVDIMM supports
persistence while backend actually failed to enable it
and used non-persistent mapping as fall back.
Instead of falling-back, QEMU should be more strict and
error out with clear message that it's not supported.
So if user asks for persistence (pmem=on), they should
store backing file on NVDIMM.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210111203332.740815-1-imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

docs/system/deprecated.rst

@@ -221,6 +221,24 @@ This machine is deprecated because we have enough AST2500 based OpenPOWER
 machines. It can be easily replaced by the ``witherspoon-bmc`` or the
 ``romulus-bmc`` machines.
 
+Backend options
+---------------
+
+Using non-persistent backing file with pmem=on (since 6.1)
+''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
+
+This option is used when ``memory-backend-file`` is consumed by emulated NVDIMM
+device. However enabling ``memory-backend-file.pmem`` option, when backing file
+is (a) not DAX capable or (b) not on a filesystem that support direct mapping
+of persistent memory, is not safe and may lead to data loss or corruption in case
+of host crash.
+Options are:
+
+    - modify VM configuration to set ``pmem=off`` to continue using fake NVDIMM
+      (without persistence guaranties) with backing file on non DAX storage
+    - move backing file to NVDIMM storage and keep ``pmem=on``
+      (to have NVDIMM with persistence guaranties).
+
 Device options
 --------------
 

util/mmap-alloc.c

@@ -225,6 +225,8 @@ static void *mmap_activate(void *ptr, size_t size, int fd,
                     "crash.\n", file_name);
             g_free(proc_link);
             g_free(file_name);
+            warn_report("Using non DAX backing file with 'pmem=on' option"
+                        " is deprecated");
         }
         /*
          * If mmap failed with MAP_SHARED_VALIDATE | MAP_SYNC, we will try