KVM是否支持vCPU热插拔？ Linux KVM客户端能否识别出新添加的vCPU？ 答案是“当然”。 像KVM内存管理一样，您可以使用“virsh”命令将vCPU添加到活动VM。 但是必须已使用Maximum vCPUs参数配置KVM guest虚拟机，则此功能将起作用。 所以在部署新的虚拟机时，您应该始终将此参数视为先决条件。如果已使用已分配的vCPU“”vcpu placement ='static' current ='N'”，所以无需为KVM guest虚拟机指定最大vCPU。

了解VM的vCPU配置:

[root@test-kvm ~]# virsh list --all
 Id    Name                           State
----------------------------------------------------
 1     generic                        running
 -     centos7                        shut off

[root@test-kvm ~]# virsh start centos7
Domain centos7 started

[root@test-kvm ~]# virsh console centos7
Connected to domain centos7

localhost login: root
Password: 
Last login: Tue Jun 13 14:03:04 on ttyS0
[root@localhost ~]# lscpu
Architecture:          x86_64
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                1
On-line CPU(s) list:   0
Thread(s) per core:    1
Core(s) per socket:    1
Socket(s):             1
NUMA node(s):          1
Vendor ID:             GenuineIntel
CPU family:            6
Model:                 61
Model name:            Intel Core Processor (Broadwell)
Stepping:              2
CPU MHz:               2400.010
BogoMIPS:              4800.02
Hypervisor vendor:     KVM
Virtualization type:   full
L1d cache:             32K
L1i cache:             32K
L2 cache:              4096K
NUMA node0 CPU(s):     0

为运行的VM增加vCPU:

接下来我们来讲vCPU数量增加到2：

[root@test-kvm ~]# virsh list --all
 Id    Name                           State
----------------------------------------------------
 1     generic                        running
 9     centos7                        running

[root@test-kvm ~]# virsh setvcpus centos7
error: command 'setvcpus' requires --count option
[root@test-kvm ~]# virsh setvcpus centos7 2

[root@test-kvm ~]# virsh dominfo centos7
Id:             9
Name:           centos7
UUID:           6693189b-0a29-4225-b822-724001270bc0
OS Type:        hvm
State:          running
CPU(s):         2
CPU time:       30.0s
Max memory:     4194304 KiB
Used memory:    2048576 KiB
Persistent:     yes
Autostart:      disable
Managed save:   no
Security model: none
Security DOI:   0

我们登录到VM验证一下：

[root@test-kvm ~]# virsh console centos7
Connected to domain centos7
Escape character is ^]  

CentOS Linux 7 (Core)
Kernel 3.10.0-514.el7.x86_64 on an x86_64

localhost login: root
Password: 
Last login: Wed Jun 14 15:01:05 on ttyS0
[root@localhost ~]# tail -f /var/log/messages 
Jun 14 15:10:52 localhost kernel: CPU1 has been hot-added
Jun 14 15:10:52 localhost kernel: SMP alternatives: switching to SMP code
Jun 14 15:10:52 localhost kernel: smpboot: Booting Node 0 Processor 1 APIC 0x1
Jun 14 15:10:52 localhost kernel: kvm-clock: cpu 1, msr 1:3ff86041, secondary cpu clock
Jun 14 15:10:52 localhost kernel: TSC synchronization [CPU#0 -> CPU#1]:
Jun 14 15:10:52 localhost kernel: Measured 1467576033050 cycles TSC warp between CPUs, turning off TSC clock.
Jun 14 15:10:52 localhost kernel: tsc: Marking TSC unstable due to check_tsc_sync_source failed
Jun 14 15:10:52 localhost kernel: KVM setup async PF for cpu 1
Jun 14 15:10:52 localhost kernel: kvm-stealtime: cpu 1, msr 13fd0f3c0
Jun 14 15:10:52 localhost kernel: Will online and init hotplugged CPU: 1
[root@localhost ~]# lscpu
Architecture:          x86_64
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                2
On-line CPU(s) list:   0,1
Thread(s) per core:    1
Core(s) per socket:    1
Socket(s):             2
NUMA node(s):          1
Vendor ID:             GenuineIntel
CPU family:            6
Model:                 61
Model name:            Intel Core Processor (Broadwell)
Stepping:              2
CPU MHz:               2400.010
BogoMIPS:              4800.02
Hypervisor vendor:     KVM
Virtualization type:   full
L1d cache:             32K
L1i cache:             32K
L2 cache:              4096K
NUMA node0 CPU(s):     0,1

为运行的VM删除VCPU

无法直接从VM中删除vCPU， 但您可以通过关闭VM后再删除vCPU；

[root@test-kvm ~]# virsh list --all
 Id    Name                           State
----------------------------------------------------
 1     generic                        running
 10    centos7                        running
[root@test-kvm ~]# virsh setvcpus centos7 1
error: unsupported configuration: failed to find appropriate hotpluggable vcpus to reach the desired target vcpu count
[root@test-kvm log]# virsh setvcpus --config centos7 1
  
[root@test-kvm log]# virsh destroy centos7
Domain centos7 destroyed

[root@test-kvm log]# virsh start centos7
Domain centos7 started

[root@test-kvm log]# virsh dominfo centos7
Id:             11
Name:           centos7
UUID:           6693189b-0a29-4225-b822-724001270bc0
OS Type:        hvm
State:          running
CPU(s):         1
CPU time:       3.3s
Max memory:     4194304 KiB
Used memory:    4194304 KiB
Persistent:     yes
Autostart:      disable
Managed save:   no
Security model: none
Security DOI:   0