[libvirt] qemu: cpu model definition problem

[Maxim Nestratov]

Hi all,

I've just encountered a problem with ability to live migrate a VM 
between hosts using the following cpu section in VM's xml:

   <cpu mode='custom' match='exact'>
     <model fallback='allow'>Haswell-noTSX</model>
     ...
     <feature policy='disable' name='rdrand'/>
   </cpu>

The problem is that a VM with such a config crashes when it migrates 
from a host where 'rdrand' presents to a host where it is absent.
This happens due to the fact that QEMU and libvirt have pretty different 
view on what 'Haswell-noTSX' is. Libvirt's description lacks some 
features, which present in QEMU description of this cpu model (e.g. 
'rdrand'). It is impossible to disable features present on the host and 
different from QEMU's ones by current libvirt code, i.e. libvirt thinks 
that no use to disable something that doesn't exist.
Of course, the problem in this case can be fixed just by adding missed 
features to 'Haswell-noTSX' model in cpu_map.xml to match what QEMU has, 
but it doesn't seem right because it will help just in this particular 
case, and in general, it will not be a solution because QEMU can change 
its cpu model definitions at any time and it does this from time to time.
What libvirt might have done is:
  - either ask QEMU for cpu model definition and work with it,
- or construct a command line for QEMU with required and disabled 
features without taking into account libvirt cpu_map.xml, i.e. always 
add '+xx' and '-yy' for features mentioned explicitly in domain xml.

What do you think?

Maxim