[libvirt] [Qemu-devel] [PATCH] 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>
> ---
> PS:
> Patch is written on top of
>   [PATCH v4 0/3] numa: deprecate '-numa node,  mem' and default memory distribution
> to avoid conflicts in qemu-deprecated.texi
>
>  numa.c               | 4 ++--
>  qemu-deprecated.texi | 8 ++++++++
>  2 files changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/numa.c b/numa.c
> index 91a29138a2..53d67b8ad9 100644
> --- a/numa.c
> +++ b/numa.c
> @@ -494,8 +494,8 @@ 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. "
> +                        "Fallback to RAM allocation is deprecated.");

Can we give the user clues on how to avoid the deprecated fallback?

Warning message nitpick: the message should be a single phrase, with no
newline or trailing punctuation.  Suggest something like

               warn_report("falling back to regular RAM allocation");
               error_printf("This is deprecated.  <Advice on what\n"
                            "to do goes here>\n");

>              /* Legacy behavior: if allocation failed, fall back to
>               * regular RAM allocation.
>               */
> diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi
> index 2fe9b72121..2193705644 100644
> --- a/qemu-deprecated.texi
> +++ b/qemu-deprecated.texi
> @@ -112,6 +112,14 @@ 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 system memory allocation from file pointed by @option{mem-path}
> +fails, QEMU fallbacks to allocating from anonymous RAM. Which might result
> +in unpredictable behavior since provided backing file wasn't used.


Noch such verb "to fallback", obvious fix "QEMU falls back to"

Suggest "RAM, which might".

Better: "since the backing file specified by the user is ignored".

>                                                                     In future
> +QEMU will not fallback and fail to start up, so user could fix his/her QEMU/host
> +configuration or explicitly use -m without -mem-path if system memory allocated
> +from anonymous RAM suits usecase.

What's "system memory allocation"?

Perhaps: "In the future, QEMU will not fall back, but fail instead.
Adjust either the host configuration [FIXME how?] or the QEMU
configuration [FIXME how?]."

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