Re: [Qemu-devel] [PATCH v4 0/9] enable numa configuration before machine_init() from QMP

[Markus Armbruster]

Igor Mammedov <address@hidden> writes:

[...]
> Series allows to configure NUMA mapping at runtime using QMP
> interface. For that to happen it introduces a new '-preconfig' CLI option
> which allows to pause QEMU before machine_init() is run and
> adds new set-numa-node QMP command which in conjunction with
> query-hotpluggable-cpus allows to configure NUMA mapping for cpus.
>
> Later we can modify other commands to run early, for example device_add.
> I recall SPAPR had problem when libvirt started QEMU with -S and, while it's
> paused, added CPUs with device_add. Intent was to coldplug CPUs (but at that
> stage it's considered hotplug already), so SPAPR had to work around the issue.

That instance is just stupidity / laziness, I think: we consider any
plug after machine creation a hot plug.  Real machines remain cold until
you press the power button.  Our virtual machines should remain cold
until they start running, i.e. with -S until the first "cont".

I vaguely remember me asking this before, but your answer didn't make it
into this cover letter, which gives me a pretext to ask again instead of
looking it up in the archives: what exactly prevents us from keeping the
machine cold enough for numa configuration until the first "cont"?

>
> Example of configuration session:
> $QEMU -smp 2 -preconfig ...
>
> QMP:
> # get CPUs layout for current target/machine/CLI
> -> {'execute': 'query-hotpluggable-cpus' }  
> <- {'return': [
>        {'props': {'core-id': 0, 'thread-id': 0, 'socket-id': 1}, ... },
>        {'props': {'core-id': 0, 'thread-id': 0, 'socket-id': 0}, ... }
>    ]}
>
> # configure 1st node
> -> {'execute': 'set-numa-node', 'arguments': { 'type': 'node', 'nodeid': 0 } 
> }  
> <- {'return': {}}
> -> {'execute': 'set-numa-node', 'arguments': { 'type': 'cpu',   
>        'node-id': 0, 'core-id': 0, 'thread-id': 0, 'socket-id': 1, }
>    }
> <- {'return': {}}
>
> # configure 2nd node
> -> {'execute': 'set-numa-node', 'arguments': { 'type': 'node', 'nodeid': 1 } }
> -> {'execute': 'set-numa-node', 'arguments': { 'type': 'cpu',  
>        'node-id': 1, 'core-id': 0, 'thread-id': 0, 'socket-id': 0 }
>    }
> <- {'return': {}}
>
> # [optional] verify configuration
> -> {'execute': 'query-hotpluggable-cpus' }  
> <- {'return': [
>        {'props': {'core-id': 0, 'thread-id': 0, 'node-id': 0, 'socket-id': 
> 1}, ... },
>        {'props': {'core-id': 0, 'thread-id': 0, 'node-id': 1, 'socket-id': 
> 0}, ... }
>    ]}
>
>
> Git tree:
>     https://github.com/imammedo/qemu.git qmp_preconfig_v3
>
> Ref to v1:
>     https://lists.gnu.org/archive/html/qemu-devel/2017-10/msg03583.html
>     Message-Id: <address@hidden>