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Re: [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())



On Thu, 13 Feb 2014 14:14:08 +0800
Chen Fan <chen fan fnst cn fujitsu com> wrote:

> On Tue, 2014-01-21 at 11:10 +0100, Andreas Färber wrote:
> > 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 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 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.
> Hi, Igor, Andreas,
> 
>   In addition, I want to know what way user could use to specify an
> arbitrary CPU if using /machine/node/N/socket/X/core/Y/thread/Z idea? 
> -device qemu64,socket=X,core=Y,thread=Z? or add a new optional command
> line?
Definitely not another CLU option.
I see a couple of options,
 1. as you suggest with additional 'numa=N' so that QEMU could know
    where to attach a new CPU.
 2. add 'parent' like option tied to link<cpu> property and specify full QOM path
    on CLI: -device cpufoo,parent=/machine/node[N]/socket[X]/...


> Thanks,
> Chen
> 
> > 
> > Regards,
> > Andreas
> > 
> 
> 



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