Re: [PATCH 0/2] RFC: add -mem-shared option

[Paolo Bonzini]

On 28/11/19 17:10, Eduardo Habkost wrote:
> On Thu, Nov 28, 2019 at 06:15:16PM +0400, Marc-André Lureau wrote:
>> Hi,
>>
>> Setting up shared memory for vhost-user is a bit complicated from
>> command line, as it requires NUMA setup such as: m 4G -object
>> memory-backend-file,id=mem,size=4G,mem-path=/dev/shm,share=on -numa
>> node,memdev=mem.
>>
>> Instead, I suggest to add a -mem-shared option for non-numa setups,
>> that will make the -mem-path or anonymouse memory shareable.
> 
> Can we make this be a "-m" option?
> 
> Or, even better: can we make "-m" options be automatically
> translated to memory-backend-* options somehow?
> 

The original idea was to always support one NUMA node, so that you could
do "-numa node,memdev=..." to specify a memory backend with -object.
However, this is not possible anymore since

    if (!mc->cpu_index_to_instance_props ||
        !mc->get_default_cpu_node_id) {
        error_setg(errp, "NUMA is not supported by this machine-type");
        return;
    }

has been added to hw/core/numa.c.

Therefore, I think instead of -mem-shared we should add a "-m
memdev=..." option.  This option:

* would be mutually exclusive with both -mem-path

* would be handled from allocate_system_memory_nonnuma.

* could be mutually exclusive "-numa node", or could just be mutually
exclusive with "-numa node,memdev=..." (the logical conclusion of that
however would be an undeprecation of "-numa node,mem=...", so that has
to be taken into account as well).

Paolo