[libvirt] [Qemu-devel] [PATCH v2] deprecate -mem-path fallback to anonymous RAM

[Markus Armbruster]

Igor Mammedov <imammedo at redhat.com> writes:

> 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 <imammedo at redhat.com>
> ---
> v2:
>  * improve text language
>     (Markus Armbruster <armbru at 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..c15e53e92d 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 or QEMU will fail to start");

The "or QEMU will fail to start" is confusing, I'm afraid.  *This* QEMU
won't.  A future QEMU may.  Suggest to simply delete the confusing part.

>              /* Legacy behavior: if allocation failed, fall back to
>               * regular RAM allocation.
>               */
> diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi
> index 2fe9b72121..1b7f3b10dc 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.
>  
> + at 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 fail to start up if RAM allocation
> +is unsuccessful.

Grammar nit: the subject of "fail to start up" is "users", oopsie.
Suggest ", and QEMU will fail to start up".

> +
>  @section QEMU Machine Protocol (QMP) commands
>  
>  @subsection block-dirty-bitmap-add "autoload" parameter (since 2.12.0)

With these tweaks:
Reviewed-by: Markus Armbruster <armbru at redhat.com>