Re: [PATCH] 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(a)redhat.com&gt; --- docs/system/deprecated.rst | 14 ++++++++++++++ util/mmap-alloc.c | 3 +++ 2 files changed, 17 insertions(+) diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst index bacd76d7a5..ba4f6ed2fe 100644 --- a/docs/system/deprecated.rst +++ b/docs/system/deprecated.rst @@ -327,6 +327,20 @@ 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 not DAX capable or 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. +Using pmem=on option with such file will return error, instead of a warning.

+Options are to move backing file to NVDIMM storage or modify VM configuration +to set ``pmem=off`` to continue using fake NVDIMM without persistence guaranties.

+ Device options -------------- diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c index 27dcccd8ec..d226273a98 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("Deprecated using non DAX backing file with" + " pmem=on option");

} /* * if map failed with MAP_SHARED_VALIDATE | MAP_SYNC,