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Re: [libvirt] [RFC] Support for CPUID masking



Speaking from an x86 angle,providing an ability to enable or disable
high level constructs like SSE instead of low level constructs, will
make it easy to understand.

Thanks

Mukesh

On Thu, Sep 3, 2009 at 3:09 PM, Daniel P. Berrange <berrange redhat com> wrote:
> On Thu, Sep 03, 2009 at 11:19:47AM +0300, Dor Laor wrote:
>> On 09/02/2009 07:09 PM, Daniel P. Berrange wrote:
>> >On Wed, Sep 02, 2009 at 11:59:39AM -0400, Jim Paris wrote:
>> >>Jiri Denemark wrote:
>> >>>Hi,
>> >>>
>> >>>We need to provide support for CPU ID masking. Xen and VMware ESX are
>> >>>examples
>> >>>of current hypervisors which support such masking.
>> >>>
>> >>>My proposal is to define new 'cpuid' feature advertised in guest
>> >>>capabilities:
>> >>...
>> >>><domain type='xen' id='42'>
>> >>>     ...
>> >>>     <features>
>> >>>         <pae/>
>> >>>         <acpi/>
>> >>>         <apic/>
>> >>>         <cpuid>
>> >>>             <mask level='1' register='ebx'>
>> >>>                 xxxx:xxxx:0000:1010:xxxx:xxxx:xxxx:xxxx
>> >>>             </mask>
>> >>...
>> >>>What are your opinions about this?
>> >>
>> >>I think it's too low-level, and the structure is x86-specific.  QEMU
>> >>and KVM compute their CPUID response based on arguments to the -cpu
>> >>argument, e.g.:
>> >>
>> >>    -cpu core2duo,model=23,+ssse3,+lahf_lm
>> >>
>> >>I think a similar structure makes more sense for libvirt, where the
>> >>configuration generally avoids big blocks of binary data, and the
>> >>XML format should suit other architectures as well.
>> >
>> >I'm going back&  forth on this too. We essentially have 3 options
>> >
>> >  - Named CPU + flags/features
>> >  - CPUID masks
>> >  - Allow either
>> >
>> >
>> >If we do either of the first two, we have to translate between the
>> >two formats for one or more of the hypervisors. For the last one we
>> >are just punting the problem off to applications.
>> >
>> >
>> >If we choose CPUID, and made QEMU driver convert to named CPU + flags
>> >we'd be stuck for non-x86 as you say.
>>
>> Why is that? cpu model + flags may apply for other arch too.
>
> If we have CPUID in the XML, there is no meaningful CPUID register
> representation for sparc/ppc/arm/etc. It is an x86 concept, which
> is almost certainly why QEMU uses named CPU models + named flags
> instead of CPUID as is public facing config.
>
> Xen/VMWare of course don't have this limitation since they only
> really care about x86.
>
> So really QEMU's  CPU model + flags approach is more generic albeit
> being much more verbose to achieve the same level of expressivity.
>
>> >If we chose named CPU + flags,  and made VMWare/Xen convert to raw
>> >CPUID we'd potentially loose information if user had defined a config
>> >with a raw CPUID mask outside context of libvirt.
>> >
>> >The other thing to remember is that CPUID also encodes sockets/cores/
>> >threads  topology data, and it'd be very desirable to expose that in
>> >a sensible fashion (ie not a bitmask).
>> >
>> >On balance i'm currently leaning to named CPU + flags + expliciti
>> >topology data because although its harder to implement for Xen/VMWare
>> >I think its much nicer to applications&  users. We might loose a tiny
>> >bit of data in the CPU ->  named/flags conversion for Xen/VMWare but
>> >I reckon we can get it good enough that most people won't really care
>> >about that.
>> >
>> >Daniel
>>
>> There are 2 more issues to consider:
>> 1. The VMW approach with all the cpuid bits might be ok, the problem is
>>    to map it into qemu model, will libvirt to that?
>
> THe problem is that CPUID is not viable for non-x86 archs so can't
> really be used as our master representation
>
>> 2. If we use the qemu approach, the host information (cpuids) need to
>>    travel to higher mgmt level in order to allow computation of
>>    greatest common denominator.
>
> Yes, whatever we decide for exposing guest CPU model/flags/etc should
> be equally applied to the libvirt capabilities XML so that apps can
> query physical host data
>
> Regards,
> Daniel
> --
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