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

[Philippe Mathieu-Daudé]

On 12/29/20 6:29 PM, Igor Mammedov wrote:
> 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 at redhat.com>
> ---
>  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

Maybe clearer enumerating? As:
"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.
> +Using pmem=on option with such file will return error, instead of a warning.

Not sure the difference between warn/err is important in the doc.

> +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.

Maybe:

The possibilities to continue using fake NVDIMM (without persistence
guaranties) are:
- move backing file to NVDIMM storage
- modify VM configuration to set ``pmem=off``

> +
>  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");

Maybe "Using non DAX backing file with 'pmem=on' option is deprecated"?

Beside the nitpicking comments,
Reviewed-by: Philippe Mathieu-Daudé <philmd at redhat.com>

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