Re: [Qemu-devel] [PATCH v2 2/2] docs: Document vCPU hotplug procedure

[Markus Armbruster]

Igor Mammedov <address@hidden> writes:

> On Tue, 25 Sep 2018 18:02:48 +0200
> Kashyap Chamarthy <address@hidden> wrote:
>
>> Signed-off-by: Kashyap Chamarthy <address@hidden>
>> ---
>>  docs/cpu-hotplug.rst | 140 +++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 140 insertions(+)
>>  create mode 100644 docs/cpu-hotplug.rst
>> 
>> diff --git a/docs/cpu-hotplug.rst b/docs/cpu-hotplug.rst
>> new file mode 100644
>> index 0000000000..8f9e63a9f7
>> --- /dev/null
>> +++ b/docs/cpu-hotplug.rst
>> @@ -0,0 +1,140 @@
>> +===================
>> +Virtual CPU hotplug
>> +===================
>> +
>> +A complete example of vCPU hotplug (and hot-unplug) using QMP
>> +``device_add`` and ``device_del``.
>> +
>> +vCPU hotplug
>> +------------
>> +
>> +(1) Launch QEMU as follows (note that the "maxcpus" is mandatory to
>> +    allow vCPU hotplug)::
>> +
>> +      $ qemu-system-x86_64 -display none -no-user-config -m 2048 \
>> +          -nodefaults -monitor stdio -machine pc,accel=kvm,usb=off \
>> +          -smp 1,maxcpus=2 -cpu IvyBridge-IBRS \
>> +          -blockdev 
>> node-name=node-Base,driver=qcow2,file.driver=file,file.filename=./base.qcow2 
>> \
>> +          -device virtio-blk,drive=node-Base,id=virtio0 -qmp 
>> unix:/tmp/qmp-sock,server,nowait
> all options beside of -smp and -qmp are not relevant here and mostly masking
> parts that matter, I'd simplify it as much as possible:
>   qemu-system-x86_64 -smp 1,maxcpus=2 -qmp unix:/tmp/qmp-sock,server,nowait 
> ...
>
> Also example only covers x86 target, but there are ppc target with spapr 
> machine (add/del)
> and s390 target (add only). probably should be mentioned here as well
>
> In addition one could coldplug cpus the same way as other devices using 
> -device CLI option.
>
>
>> +(2) Run 'qmp-shell' (located in the source tree) to connect to the
>> +    just-launched QEMU::
>> +
>> +      $> ./qmp/qmp-shell -p -v /tmp/qmp-sock
>> +      [...]
>> +      (QEMU)
>> +
>> +(3) Check which socket is free to allow hotplugging a CPU::
> may be: which cpus are possible to plug (an entry with qom-path property 
> describes an existing cpu)

Suggest

    (3) Find out which CPU types could be plugged, and into which sockets:

>> +
>> +      (QEMU) query-hotpluggable-cpus
>> +      {
>> +          "execute": "query-hotpluggable-cpus",
>> +          "arguments": {}
>> +      }
>> +      {
>> +          "return": [
>> +              {
>> +                  "type": "IvyBridge-IBRS-x86_64-cpu",
>> +                  "vcpus-count": 1,
>> +                  "props": {
>> +                      "socket-id": 1,
>> +                      "core-id": 0,
>> +                      "thread-id": 0
>> +                  }
>> +              },
>> +              {
>> +                  "qom-path": "/machine/unattached/device[0]",
>> +                  "type": "IvyBridge-IBRS-x86_64-cpu",
>> +                  "vcpus-count": 1,
>> +                  "props": {
>> +                      "socket-id": 0,
>> +                      "core-id": 0,
>> +                      "thread-id": 0
>> +                  }
>> +              }
>> +          ]
>> +      }
>> +      (QEMU)
>> +
>> +(4) We can see that socket 1 is free,

How?  I know, but only because I just read the documentation of
query-hotpluggable-cpus.  Which by the way sucks.  For instance, will
the command always return exactly one HotpluggableCPU object per socket?
Anyway, what about this:

    The command returns an object with a "qom-path" member for each
    present CPU.  In this case, it shows an IvyBridge-IBRS-x86_64-cpu in
    socket 0.

    It returns an object without a "qom-path" for every possibly CPU
    hot-plug.  In this case, it shows you can plug an
    IvyBridge-IBRS-x86_64-cpu into socket 1, and the additional
    properties you need to pass to device_add for that.

>>                                        so use `device_add` to hotplug
> ... and 'arguments' provide a list of property/value pairs to create 
> corresponding cpu.
>
>> +    "IvyBridge-IBRS-x86_64-cpu"::

Suggest

    (4) Hot-plug an additional CPU:

>> +
>> +      (QEMU) device_add id=cpu-2 driver=IvyBridge-IBRS-x86_64-cpu 
>> socket-id=1 core-id=0 thread-id=0
>> +      {
>> +          "execute": "device_add",
>> +          "arguments": {
>> +              "socket-id": 1,
>> +              "driver": "IvyBridge-IBRS-x86_64-cpu",
>> +              "id": "cpu-2",
>> +              "core-id": 0,
>> +              "thread-id": 0
>> +          }
>> +      }
>> +      {
>> +          "return": {}
>> +      }
>> +      (QEMU)
>> +
[...]