[libvirt] [RFC] Support for CPUID masking

Sun Oct 11 09:44:04 UTC 2009

On 10/09/2009 12:58 PM, Mukesh G wrote:
> 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.

ain't SSE a low level construct too?

What about another approach for the cpuid issue:
I think that dealing with specific flags is pretty error prone on all 
levels - virt-mgr, libvirt, qemu, migration, and even the guest.
We can't just 'invent' new cpu modules that will be a combination of 
flags we have on the host. It might work for some kernels/apps but it 
might break others. In addition to cpuid we have the stepping and more 
MSRs that needed to be hidden/exposed.

The offer:
Libvirt, virt-mgr, qemu will not deal with lowlevel bits of the cpuid.
We'll use predefined cpu modules to be emulated.
These predefined modules will represent a real module that was once 
shipped by Intel/AMD.

We'll write an additional tool, not under libvirt, that will be able to 
calculate all the virtual cpus that can run over the physical cpu. This 
tool will be the one messing with /proc/cpuinfo, etc.

Example (theoretical): Physical cpu is "Intel(R) Core(TM)2 Duo CPU 
T9600  @ 2.80GHz" the output of the tool will be:
shell>computeVirtCpuCapabilities
core2duo
core solo
486
pentium3
..

Libvirt will only expose the real physical cpu and all of the outputs 
above. Higher level mgmt will compute the best -cpu to pick for the VM, 
and it will take account the user needs for performance or for migration 
flexibility.

The cons:
  - Simple for all levels
  - Fast implementation
  - Accurate representative of real cpus

The pros:
  - none - we should write the tool anyway
  - Maybe we hide some of the possibilities, but as said above, it is
    safer and friendlier.
    I also recommend of adding a field to be added next to the cpu for
    flexibility and possible future changes + dealing with problematic
    bios with NX bit disabled.

Regards,
Dor

>
> Thanks
>
> Mukesh
>
> On Thu, Sep 3, 2009 at 3:09 PM, Daniel P. Berrange<berrange at 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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