[libvirt] [Qemu-devel] Exposing and calculating CPU APIC IDs (was Re: [RFC 1/3] target-i386: moving registers of vmstate from cpu_exec_init() to x86_cpu_realizefn())

Andreas Färber afaerber at suse.de
Tue Jan 21 10:10:30 UTC 2014

Am 21.01.2014 10:51, schrieb Chen Fan:
> On Tue, 2014-01-21 at 10:31 +0100, Igor Mammedov wrote:
>> On Tue, 21 Jan 2014 15:12:45 +0800
>> Chen Fan <chen.fan.fnst at cn.fujitsu.com> wrote:
>>> On Mon, 2014-01-20 at 13:29 +0100, Igor Mammedov wrote:
>>>> On Fri, 17 Jan 2014 17:13:55 -0200
>>>> Eduardo Habkost <ehabkost at redhat.com> wrote:
>>>>> On Wed, Jan 15, 2014 at 03:37:04PM +0100, Igor Mammedov wrote:
>>>>>> I recall there were objections to it since APIC ID contains topology
>>>>>> information and it's not trivial for user to get it right.
>>>>>> The last idea that was discussed to fix it was not expose APIC ID to
>>>>>> user but rather introduce QOM hierarchy like:
>>>>>>   /machine/node/N/socket/X/core/Y/thread/Z
>>>>>> and use it in user interface as a means to specify an arbitrary CPU
>>>>>> and let QEMU calculate APIC ID based on this path.
>>>>>> But nobody took on implementing it yet.
>>>>> We're taking so long to get a decent interface implemented, that part of
>>>>> me is considering exposing the APIC ID directly like suggested before,
>>>>> and requiring libvirt to calculate topology-aware APIC IDs[1] to
>>>>> properly implement CPU hotplug (and possibly for other tasks).
>>>> If you are speaking about 
>>>> 'qemu will core dump with "-smp 254, sockets=2, cores=3, threads=2"'
>>>> http://patchwork.ozlabs.org/patch/301272/
>>>> bug then it's limitation of ACPI implementation,
>>>> I'm going to refactor it to use full APIC ids instead of using bitmap,
>>>> so that we won't ever run into issue regardless of cpu supported CPU count.
>>>>> Another part of me is hoping that the libvirt developers ask us to
>>>>> please not do that, so I can use it as argument against exposing the
>>>>> APIC IDs directly the next time we discuss this.  :)
>>>> why not try your  /machine/node/N/socket/X/core/Y/thread/Z idea first.
>>>> It will benefit not only cpu hotplug but also '-numa' and topology
>>>> description in general.
>>> have there been any plan/model of the idea? Need to add a new option to
>>> qemu command?
>> I suppose we can start with internal default implementation first.
>> one way could be
>>  1. let machine prebuild empty QOM tree /machine/node/N/socket/X/core/Y/thread/Z
>>  2. add node, socket, core, thread properties to CPU and link CPU into respective
>>     link created by #1
> Thanks, I hope I can take some time to make some patches to implement
> it.

Please give us a few hours to reply. :)

/machine/node seems too broad a term to me.
You can't prebuild the full tree, you can only prepare the nodes.
core[Y]/thread[Z] was previously discussed as syntax.

The important part to decide on will be what is going to be child<> and
what link<>. Has anyone played with the Intel Quark platform for
instance? (Galileo board or upcoming Edison card) On a regular
mainboard, we would have socket[X] as a link<x86_64-cpu>, which might
point to a child<cpu> /machine/memory-node[W]/cpu[X]. But if we do so we
can't reassign it to another memory node - acceptable? With Quark (or
Qseven modules etc.) there would be a container object rather than the
/machine itself that has a child<i386-cpu> instead of a link<i386-cpu>.
I guess the memory nodes could still be on the /machine though.
The other point of discussion between Anthony and me was whether core[Y]
should be a link<> or child<>, same for thread. I believe a child<> is
better as it enforces that unrealizing the CPU will unrealize all its
cores and all its threads in the future.

More issues may pop up when thinking about it longer than a few minutes.
But yes, we need to start investigating this, and so far I had other
priorities like getting the CPUState mess I created cleaned up.


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