If I run the dual core inner VM via `taskset 0x03 qemu-system-riscv64
...`, the amd64 cores are released while the inner VM idles. But once
the inner VM is stopped the amd64 CPU consumption sticks at 2 CPUs.

`mpstat -P ALL` provides this output
(https://paste.ubuntu.com/p/5ZDbDqJ86g/):


    07:38:56 PM  CPU    %usr   %nice    %sys %iowait    %irq   %soft  %steal  
%guest  %gnice   %idle
    07:38:56 PM  all    1.66    0.00    2.11    0.04    0.00    0.01    0.00    
3.53    0.00   92.66
    07:38:56 PM    0    1.99    0.00    5.88    0.04    0.00    0.01    0.00   
28.11    0.00   63.98
    07:38:56 PM    1    6.10    0.00   10.23    0.10    0.00    0.05    0.00   
28.60    0.00   54.93
    07:38:56 PM    2    0.89    0.00    1.63    0.01    0.00    0.01    0.00    
0.00    0.00   97.46
    07:38:56 PM    3    1.60    0.00    1.40    0.01    0.00    0.01    0.00    
0.00    0.00   96.98
    07:38:56 PM    4    0.94    0.01    1.13    0.07    0.00    0.02    0.00    
0.00    0.00   97.82
    07:38:56 PM    5    1.17    0.00    0.88    0.05    0.00    0.01    0.00    
0.00    0.00   97.89
    07:38:56 PM    6    0.82    0.00    1.04    0.01    0.00    0.01    0.00    
0.00    0.00   98.12
    07:38:56 PM    7    0.90    0.00    0.80    0.02    0.00    0.01    0.00    
0.00    0.00   98.27
    07:38:56 PM    8    2.50    0.00    1.28    0.03    0.00    0.01    0.00    
0.00    0.00   96.18
    07:38:56 PM    9    1.21    0.00    2.25    0.01    0.00    0.01    0.00    
0.00    0.00   96.52
    07:38:56 PM   10    0.90    0.00    0.90    0.01    0.00    0.01    0.00    
0.00    0.00   98.19
    07:38:56 PM   11    2.06    0.00    1.66    0.18    0.00    0.01    0.00    
0.00    0.00   96.09
    07:38:56 PM   12    1.06    0.00    0.98    0.01    0.00    0.01    0.00    
0.00    0.00   97.94
    07:38:56 PM   13    0.98    0.00    1.48    0.02    0.00    0.01    0.00    
0.00    0.00   97.51
    07:38:56 PM   14    1.81    0.00    1.01    0.02    0.00    0.01    0.00    
0.00    0.00   97.14
    07:38:56 PM   15    1.57    0.00    1.25    0.02    0.00    0.02    0.00    
0.00    0.00   97.14

Column %guest shows the percentage of time spent by the CPUs to run a
virtual processor.

Though the inner VM is stopped the VCPUs don't idle completely.

Idling is implemented via the `wfi` opcode. Could it be that the cores
assigned to VCPUs still receive abundant interrupts?

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https://bugs.launchpad.net/bugs/2112578

Title:
  riscv64: KVM does not release harts

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