On Mon, 11 Jan 2021 15:33:32 -0500
Igor Mammedov <imamm...@redhat.com> 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 <imamm...@redhat.com>
> Reviewed-by: Philippe Mathieu-Daudé <phi...@redhat.com>
> ---
> v2:
> rephrase deprecation comment andwarning message
> (Philippe Mathieu-Daudé <phi...@redhat.com>)
I've posted as v1 though it's v2 and it looks like it fell through cracks,
can someone pick it up if it looks fine, please?
> ---
> 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,
