On 6/26/19 9:42 AM, Igor Mammedov wrote:
> Fallback might affect guest or worse whole host performance
> or functionality if backing file were used to share guest RAM
> with another process.
>
> Patch deprecates fallback so that we could remove it in future
> and ensure that QEMU will provide expected behavior and fail if
> it can't use user provided backing file.
>
> Signed-off-by: Igor Mammedov <imamm...@redhat.com>
> Reviewed-by: Daniel P. Berrangé <berra...@redhat.com>
> Reviewed-by: Markus Armbruster <arm...@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <phi...@redhat.com>
> ---
> v2:
> * improve text language
> (Markus Armbruster <arm...@redhat.com>)
> v3:
> * drop confusing 'or QEMU will fail to start' in error message
> * fix up grammar error
> (Markus Armbruster <arm...@redhat.com>)
>
> numa.c | 6 ++++--
> qemu-deprecated.texi | 9 +++++++++
> 2 files changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/numa.c b/numa.c
> index 91a29138a2..1994ab0655 100644
> --- a/numa.c
> +++ b/numa.c
> @@ -494,8 +494,10 @@ static void allocate_system_memory_nonnuma(MemoryRegion
> *mr, Object *owner,
> if (mem_prealloc) {
> exit(1);
> }
> - error_report("falling back to regular RAM allocation.");
> -
> + warn_report("falling back to regular RAM allocation");
> + error_printf("This is deprecated. Make sure that -mem-path "
> + " specified path has sufficient resources to
> allocate"
> + " -m specified RAM amount");
> /* Legacy behavior: if allocation failed, fall back to
> * regular RAM allocation.
> */
> diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi
> index 2fe9b72121..9cba82d5ec 100644
> --- a/qemu-deprecated.texi
> +++ b/qemu-deprecated.texi
> @@ -112,6 +112,15 @@ QEMU using implicit generic or board specific splitting
> rule.
> Use @option{memdev} with @var{memory-backend-ram} backend or @option{mem} (if
> it's supported by used machine type) to define mapping explictly instead.
>
> +@subsection -mem-path fallback to RAM (since 4.1)
> +Currently if guest RAM allocation from file pointed by @option{mem-path}
> +fails, QEMU falls back to allocating from RAM, which might result
> +in unpredictable behavior since the backing file specified by the user
> +is ignored. In the future, users will be responsible for making sure
> +the backing storage specified with @option{-mem-path} can actually provide
> +the guest RAM configured with @option{-m} and QEMU will fail to start up if
> +RAM allocation is unsuccessful.
> +
> @section QEMU Machine Protocol (QMP) commands
>
> @subsection block-dirty-bitmap-add "autoload" parameter (since 2.12.0)
>
