Re: [Qemu-devel] [PATCH v7] docs: add cpu-hotplug.txt

[Dou Liyang <douly.fnst@cn.fujitsu.com>]

Hi, Eduardo

At 09/07/2016 04:05 AM, Eduardo Habkost wrote:
> On Tue, Aug 23, 2016 at 01:17:01PM +0800, Dou Liyang wrote:
>> Hi Alexandre,
>>
>> At 08/22/2016 04:56 PM, Alexandre DERUMIER wrote:
>>> Hello,
>>>
>>> I'm looking to implement cpu hotplug,
>>>
>>> and I have a question about cpu flags
>>>
>>> currently I have something like
>>>
>>> -cpu qemu64,+lahf_lm,+sep,+kvm_pv_unhalt,+kvm_pv_eoi,enforce
>>> -smp 4,sockets=2,cores=2,maxcpus=4
>>>
>>>
>>> Does I need to define flags like:
>>>
>>> -smp 2,sockets=2,cores=2,maxcpus=4
>>> -device qemu64-x86_64-cpu,+lahf_lm,+sep,+kvm_pv_unhalt,+kvm_pv_eoi,enforce,id=cpu1,socket-id=1,core-id=1,thread-id=0
>>
>> I think we don't need to do that.
>> In my option, just like this:
>>
>> -device qemu64-x86_64-cpu,id=cpu1,socket-id=1,..
>>
>> Because QEMU sets the "-cpu" options in MachineState:
>>
>>   current_machine->cpu_model = cpu_model;
>>
>> when you add a CPU, QEMU can get the flag from the MachineState.
>
> You don't need to repeat the flags, but that's not because of
> MachineState::cpu_model, but because of the semantics of -smp:

Sorry for the reply.

>
> The option:
>   -smp MODEL,+FOO,+BAR

I guess you may mean "-cpu", not "-smp"

> is internally translated to:
>   -global MODEL.FOO=on
>   -global MODEL.BAR=on
> in addition to setting the CPU model for creating initial CPUs to
> MODEL.
>

I see.

>>
>>> ...
>>>
>>> ?
>>>
>>>
>>> Another question,
>>> is -smp mandatory ?  (if I want coldplug all cpus)
>>
>> it's not mandatory. such as this:
>>
>>   ./x86_64-softmmu/qemu-system-x86_64 -m 1G /image/fedora.img
>>   -enable-kvm -monitor stdio
>>
>>  (qemu) info cpus
>>   * CPU #0: pc=0xffffffff81060586 (halted) thread_id=4032
>>
>> the default number of CPUs is 1.
>>
>>>
>>> -smp sockets=2,cores=2,maxcpus=4
>>
>>> -device qemu64-x86_64-cpu,id=cpu1,socket-id=1,core-id=1,thread-id=0
>>> -device qemu64-x86_64-cpu,id=cpu1,socket-id=1,core-id=2,thread-id=0
>>> -device qemu64-x86_64-cpu,id=cpu3,socket-id=2,core-id=1,thread-id=0
>>> -device qemu64-x86_64-cpu,id=cpu4,socket-id=2,core-id=2,thread-id=0
>>>
>>> or does I need minimum 1 non unplugable cpu
>
> As mentioned above, the default number of CPUs is 1, so "-smp
> sockets=2" is the same as "-smp 1,sockets=2".
>
> I assume you mean something like "-smp 0", but that doesn't work
> today. I would like to eventually allow all CPUs to be created
> using -device, but that's not possible yet.

I am interested in what is the benefit, if we can create all CPUs using
-device.   :)


Thanks,
Dou
>
>>>
>>> -smp 1,sockets=2,cores=2,maxcpus=4
>>> -device qemu64-x86_64-cpu,id=cpu1,socket-id=1,core-id=2,thread-id=0
>>> -device qemu64-x86_64-cpu,id=cpu3,socket-id=2,core-id=1,thread-id=0
>>> -device qemu64-x86_64-cpu,id=cpu4,socket-id=2,core-id=2,thread-id=0
>>>
>>
>> I think that is better, and the socket-id/core-id/thread-id starts at
>> index 0
>>
>> I am new to the community. Please don't mind, and take with a grain of
>> salt.	
>>
>> Thanks,
>> Dou
>>
>>
>