Why can not get the expected cpu topology after setting the maxcpu in xml?

[liang chaojun]

Hello all,

I hit a issue that I'm not able to see the expected cpu topology in VM after 
setting the maxcpu arrtribute in domain.

the xml look like below:
<vcpu placement='static' current='4'>8</vcpu>
  <cpu mode='host-passthrough' check='none' migratable='on'>
    <topology sockets='2' dies='1' cores='4' threads='1'/>
  </cpu>

I notice that the cpu topoloy looked like below capture output from 'lscpu' 
tool which is totally different with my setting.

CPU(s):                4
On-line CPU(s) list:   0-3
Thread(s) per core:    1
Core(s) per socket:    4
Socket(s):             1
NUMA node(s):          1
Vendor ID:             GenuineIntel
CPU family:            6
Model:                 63
Model name:            Intel(R) Xeon(R) CPU E5-2699 v3 @ 2.30GHz

Besides i found it looked correct if i relpace 'vcpu' line with "<vcpu 
placement='static'>8</vcpu>" that remove maxcpu value.

Could you give me some information? Any comments are appreciated.

Thanks