Re: [PATCH] qemu-options.hx: Update descriptions of memory options for NUMA node

[Alex Bennée]

Yohei Kojima <y-koj@outlook.jp> writes:

> This commit adds the following description:
> 1. `memdev` option is recommended over `mem` option (see [1,2])
> 2. users must specify memory for all NUMA nodes (see [2])
>
> This commit also separates descriptions for `mem` and `memdev` into two
> paragraphs. The old doc describes legacy `mem` option first, and it was
> a bit confusing.
>
> Related documantations:
> [1] https://wiki.qemu.org/ChangeLog/5.1#Incompatible_changes
> [2] https://www.qemu.org/docs/master/about/removed-features.html
>
> Signed-off-by: Yohei Kojima <y-koj@outlook.jp>
> ---
>  qemu-options.hx | 25 ++++++++++++++++---------
>  1 file changed, 16 insertions(+), 9 deletions(-)
>
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 59bdf67a2c..174f0d0c2d 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -405,15 +405,22 @@ SRST
>          -numa node,nodeid=0 -numa node,nodeid=1 \
>          -numa cpu,node-id=0,socket-id=0 -numa cpu,node-id=1,socket-id=1
>  
> -    Legacy '\ ``mem``\ ' assigns a given RAM amount to a node (not supported
> -    for 5.1 and newer machine types). '\ ``memdev``\ ' assigns RAM from
> -    a given memory backend device to a node. If '\ ``mem``\ ' and
> -    '\ ``memdev``\ ' are omitted in all nodes, RAM is split equally between 
> them.
> -
> -
> -    '\ ``mem``\ ' and '\ ``memdev``\ ' are mutually exclusive.
> -    Furthermore, if one node uses '\ ``memdev``\ ', all of them have to
> -    use it.
> +    '\ ``memdev``\ ' option assigns RAM from a given memory backend
> +    device to a node. It is recommended to use '\ ``memdev``\ ' option
> +    over legacy '\ ``mem``\ ' option. This is because '\ ``memdev``\ '
> +    option provides better performance and more control over the
> +    backend's RAM (e.g. '\ ``prealloc``\ ' parameter of
> +    '\ ``-memory-backend-ram``\ ' allows memory preallocation).
> +
> +    For compatibility reasons, legacy '\ ``mem``\ ' option is
> +    supported in 5.0 and older machine types. Note that '\ ``mem``\ '
> +    and '\ ``memdev``\ ' are mutually exclusive. If one node uses
> +    '\ ``memdev``\ ', the rest nodes have to use '\ ``memdev``\ '
> +    option, and vice versa.
> +
> +    Users must specify memory for all NUMA nodes by '\ ``memdev``\ '
> +    (or legacy '\ ``mem``\ ' if available). In QEMU 5.2, the support
> +    for '\ ``-numa node``\ ' without memory specified was removed.

I think this mixes up memdev and mem too much. It would be better to
make the lead up to the example just talk about memdev (as it is the
preferred option) and move the discussion about backwards compatibility
to after the example. You can use the .. note:: annotation to put it in
a nice little box, something like:

.. note::

    For compatibility reasons, legacy '\ ``mem``\ ' option is
    supported in 5.0 and older machine types. Note that '\ ``mem``\ '
    and '\ ``memdev``\ ' are mutually exclusive. If one node uses '\
    ``memdev``\ ', the rest of the nodes must also use the '\
    ``memdev``\ ' option, and vice versa.


>  
>      '\ ``initiator``\ ' is an additional option that points to an
>      initiator NUMA node that has best performance (the lowest latency or


-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro