[PATCH] Deprecate pmem=on with non-DAX capable backend file

[Igor Mammedov <imammedo@xxxxxxxxxx>]

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@xxxxxxxxxx>
Reviewed-by: Philippe Mathieu-Daudé <philmd@xxxxxxxxxx>
---
v2:
  rephrase deprecation comment andwarning message
  (Philippe Mathieu-Daudé <philmd@xxxxxxxxxx>)
---
 docs/system/deprecated.rst | 17 +++++++++++++++++
 util/mmap-alloc.c          |  3 +++
 2 files changed, 20 insertions(+)

diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst
index bacd76d7a5..e79fb02b3a 100644
--- a/docs/system/deprecated.rst
+++ b/docs/system/deprecated.rst
@@ -327,6 +327,23 @@ The Raspberry Pi machines come in various models (A, A+, B, B+). To be able
 to distinguish which model QEMU is implementing, the ``raspi2`` and ``raspi3``
 machines have been renamed ``raspi2b`` and ``raspi3b``.
 
+Backend options
+---------------
+
+Using non-persistent backing file with pmem=on (since 6.0)
+''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
+
+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
 --------------
 
diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
index 27dcccd8ec..0388cc3be2 100644
--- a/util/mmap-alloc.c
+++ b/util/mmap-alloc.c
@@ -20,6 +20,7 @@
 #include "qemu/osdep.h"
 #include "qemu/mmap-alloc.h"
 #include "qemu/host-utils.h"
+#include "qemu/error-report.h"
 
 #define HUGETLBFS_MAGIC       0x958458f6
 
@@ -166,6 +167,8 @@ void *qemu_ram_mmap(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 map failed with MAP_SHARED_VALIDATE | MAP_SYNC,
-- 
2.27.0