在KVM环境运行Windows 10，发现性能非常差，在物理主机上检查
top - 13:43:41 up 6 days, 22:24, 3 users, load average: 3.15, 2.72, 2.01
Tasks: 205 total, 4 running, 201 sleeping, 0 stopped, 0 zombie
%Cpu(s): 49.8 us, 20.2 sy, 0.0 ni, 29.7 id, 0.3 wa, 0.0 hi, 0.1 si, 0.0 st
KiB Mem : 8113620 total, 132176 free, 6632048 used, 1349396 buff/cache
KiB Swap: 999420 total, 32 free, 999388 used. 1018384 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
7656 libvirt+ 20 0 7248848 3.720g 2512 S 146.2 48.1 736:00.16 qemu-system-x86
21715 libvirt+ 20 0 6707652 2.873g 2704 R 128.2 37.1 145:05.49 qemu-system-x86
31 root 25 5 0 0 0 S 1.6 0.0 133:51.26 ksmd
22127 huatai 20 0 41936 3584 2972 R 1.6 0.0 0:00.30 top
其中，7656进程对应的是win2106虚拟机，在虚拟机内部观察cpu繁忙程度只有 80%，但是物理服务器上显示则占用cpu资源翻倍，约170%。（仅仅是更新操作系统）
win10虚拟机则更为夸张，没有任何程序运行，在虚拟机内部观察CPU几乎是空闲的，但是远程操作依然非常缓慢，在物理服务器上观察进程 21715 占用CPU资源超过130%。
此外观察可以看到，系统的sys占用非常严重，表明虚拟化消耗了很多系统资源
top - 14:07:23 up 6 days, 22:48, 3 users, load average: 3.25, 3.05, 2.86
Tasks: 206 total, 3 running, 203 sleeping, 0 stopped, 0 zombie
%Cpu0 : 46.5 us, 14.7 sy, 0.0 ni, 38.8 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu1 : 40.8 us, 31.7 sy, 0.0 ni, 26.8 id, 0.0 wa, 0.0 hi, 0.7 si, 0.0 st
%Cpu2 : 40.1 us, 20.1 sy, 0.0 ni, 39.8 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu3 : 70.3 us, 20.3 sy, 0.0 ni, 9.1 id, 0.3 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem : 8113620 total, 142212 free, 6650924 used, 1320484 buff/cache
KiB Swap: 999420 total, 0 free, 999420 used. 999268 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
7656 libvirt+ 20 0 7248848 3.732g 2504 S 146.4 48.2 770:42.78 qemu-system-x86
21715 libvirt+ 20 0 6707652 2.864g 2664 R 131.1 37.0 177:21.19 qemu-system-x86
31 root 25 5 0 0 0 S 4.6 0.0 135:19.57 ksmd
22193 huatai 20 0 41936 3712 3016 R 0.7 0.0 0:00.19 top
43 root 20 0 0 0 0 R 0.3 0.0 3:25.96 kswapd0
host主机messages日志可以观察到
[592420.654337] kvm [21715]: vcpu1 unhandled rdmsr: 0x611
[592427.561224] kvm [21715]: vcpu0 unhandled rdmsr: 0x606
升级了物理主机系统，然后重启，只运行一个win10系统，但是依然显示虚拟机占用资源很高。看上去sys是罪魁祸首。
top - 15:18:49 up 54 min, 1 user, load average: 1.44, 0.94, 0.41
Tasks: 193 total, 1 running, 192 sleeping, 0 stopped, 0 zombie
%Cpu0 : 13.1 us, 11.4 sy, 0.0 ni, 75.4 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu1 : 18.9 us, 30.4 sy, 0.0 ni, 50.7 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu2 : 7.9 us, 5.9 sy, 0.0 ni, 86.2 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu3 : 13.1 us, 28.2 sy, 0.0 ni, 58.7 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem : 8113624 total, 3713084 free, 3862496 used, 538044 buff/cache
KiB Swap: 999420 total, 999420 free, 0 used. 3956908 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
3072 libvirt+ 20 0 6321552 3.232g 22936 S 131.9 41.8 6:28.02 qemu-system-x86
31 root 25 5 0 0 0 S 0.3 0.0 0:01.17 ksmd
1245 root 20 0 669544 46288 32152 S 0.3 0.6 0:08.28 dockerd
3121 huatai 20 0 41936 3896 3168 R 0.3 0.0 0:00.09 top
1 root 20 0 38148 6232 4032 S 0.0 0.1 0:03.63 systemd
2 root 20 0 0 0 0 S 0.0 0.0 0:00.00 kthreadd
参考 High KVM/QEMU CPU utilization when Windows 10 guest is idle 可以采用perf来检查qemu消耗cpu时间的原因，检查是在内核空间、用户空间还是guest空间消耗的cpu
sudo perf kvm --host top -p `pidof qemu-system-x86_64`Samples: 304K of event 'cycles:pp', Event count (approx.): 160577711871554
Overhead Shared Object Symbol
38.52% [kernel] [k] poll_freewait
5.57% [kernel] [k] do_sys_poll
4.87% [kernel] [k] vmx_vcpu_run
4.82% [kernel] [k] mutex_lock_killable
4.07% libglib-2.0.so.0.4800.2 [.] g_main_context_check
3.63% libpthread-2.23.so [.] pthread_mutex_lock
2.89% qemu-system-x86_64 [.] aio_notify_accept
2.67% [kernel] [k] enqueue_entity
2.62% qemu-system-x86_64 [.] qemu_notify_event
2.02% libpthread-2.23.so [.] pthread_mutex_unlock
1.69% [kernel] [k] native_write_msr_safe
1.65% [kernel] [k] __x86_indirect_thunk_rax
0.95% [kernel] [k] kvm_get_apic_interrupt
0.95% [kernel] [k] __fget
0.93% [kernel] [k] kvm_fetch_guest_virt
Samples: 331K of event 'cycles:pp', Event count (approx.): 200290188293710
Overhead Shared Object Symbol
35.14% [kernel] [k] sock_poll
8.07% libglib-2.0.so.0.4800.2 [.] g_main_context_prepare
7.21% [kernel] [k] check_preempt_wakeup
6.59% libpthread-2.23.so [.] pthread_mutex_lock
5.53% [kernel] [k] __srcu_read_lock
4.80% [kernel] [k] vmx_vcpu_run
Samples: 350K of event 'cycles:pp', Event count (approx.): 219305848967883
Overhead Shared Object Symbol
18.52% libglib-2.0.so.0.4800.2 [.] 0x00000000000473b0
15.54% [kernel] [k] update_cfs_shares
11.52% qemu-system-x86_64 [.] aio_pending
9.07% [kernel] [k] sock_poll
5.88% qemu-system-x86_64 [.] timerlistgroup_deadline_ns
5.63% [kernel] [k] vmx_vcpu_run
2.83% [kernel] [k] eventfd_poll
2.35% [kernel] [k] do_sys_poll
2.26% qemu-system-x86_64 [.] qemu_mutex_lock
1.72% libpthread-2.23.so [.] pthread_mutex_lock
Samples: 431K of event 'cycles:pp', Event count (approx.): 200377848626454
Overhead Shared Object Symbol
26.47% qemu-system-x86_64 [.] qemu_mutex_unlock
23.95% qemu-system-x86_64 [.] aio_pending
6.31% [kernel] [k] vmx_vcpu_run
4.96% libglib-2.0.so.0.4800.2 [.] g_main_context_check
4.75% [kernel] [k] __x86_indirect_thunk_rax
3.75% [kernel] [k] entry_SYSCALL_64_fastpath
3.45% [kernel] [k] __gfn_to_hva_many
其中，如果是 [k] 开头则是kernel space，如果是 [.] 开头则是user space。
如果VM大量执行VM_EXIT则需要调查为何会如尝试
perf stat -e 'kvm:*' -a -- sleep 1
perf kvm --host stat live这个方式观察主机非常方便
perf kvm是特定指令，针对kvm进行性能分析
发现一个问题，在执行 perf kvm --host stat live 时发现，大量都VM-EXIT工作都消耗在IO_INSTRUCTION，并且有大量的MSR_READ和MSR_WRITE
15:52:35.709821
Analyze events for all VMs, all VCPUs:
VM-EXIT Samples Samples% Time% Min Time Max Time Avg time
IO_INSTRUCTION 55654 78.67% 42.39% 3.54us 74.94us 13.63us ( +- 0.24% )
MSR_READ 7114 10.06% 0.74% 1.46us 16.58us 1.87us ( +- 0.30% )
MSR_WRITE 3781 5.34% 0.61% 2.09us 9.84us 2.88us ( +- 0.35% )
HLT 2933 4.15% 56.06% 1.71us 15227.79us 341.97us ( +- 8.91% )
EXTERNAL_INTERRUPT 731 1.03% 0.11% 1.19us 7.07us 2.63us ( +- 0.95% )
PENDING_INTERRUPT 413 0.58% 0.05% 1.32us 2.96us 2.25us ( +- 0.50% )
TPR_BELOW_THRESHOLD 82 0.12% 0.01% 2.54us 3.69us 2.93us ( +- 1.12% )
EPT_VIOLATION 26 0.04% 0.01% 3.81us 36.55us 8.78us ( +- 15.27% )
EPT_MISCONFIG 7 0.01% 0.01% 12.27us 39.47us 21.80us ( +- 17.78% )
PAUSE_INSTRUCTION 5 0.01% 0.00% 1.61us 1.99us 1.81us ( +- 4.30% )
Total Samples:70746, Total events handled time:1789115.74us.
在原文中建议，如果出现大量的VM_EXIT，则有可能是由于APIC时钟中断（timer interrupts）导致的kvm_apic_accept_irq。需要检查Windows 10是否使用APIC时钟。hyperv hypercall可以使用MSR，这样kvm_msr可能会被hyperv synthetic中断时钟调用 1000次/s。
有可能虚拟机在不断唤醒并执行halt polling，导致消耗了主机大量cpu。
在 KVM: High host CPU load after upgrading VM to windows 10 1803 提到了解决方法：需要在KVM虚拟机配置中添加hpet时钟。
检查当前默认配置：
<clock offset='localtime'>
<timer name='rtc' tickpolicy='catchup'/>
<timer name='pit' tickpolicy='delay'/>
<timer name='hpet' present='no'/>
<timer name='hypervclock' present='yes'/>
</clock>修改成
<clock offset='localtime'>
<timer name='rtc' tickpolicy='catchup'/>
<timer name='pit' tickpolicy='delay'/>
<timer name='hpet' present='yes'/>
<timer name='hypervclock' present='yes'/>
</clock>果然修改以后再次启动windows虚拟机，可以看到sys负载已经极低，并且
top - 19:24:56 up 5:00, 2 users, load average: 0.55, 0.41, 0.19
Tasks: 197 total, 1 running, 196 sleeping, 0 stopped, 0 zombie
%Cpu0 : 1.3 us, 0.7 sy, 0.0 ni, 98.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu1 : 1.0 us, 0.0 sy, 0.0 ni, 98.7 id, 0.3 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu2 : 3.3 us, 0.3 sy, 0.0 ni, 96.3 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu3 : 0.7 us, 0.0 sy, 0.0 ni, 99.3 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem : 8113624 total, 3432512 free, 4108152 used, 572960 buff/cache
KiB Swap: 999420 total, 999420 free, 0 used. 3702616 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
3651 libvirt+ 20 0 7454076 3.456g 23040 S 6.6 44.7 3:55.88 qemu-system-x86
3663 huatai 20 0 41800 3760 3108 R 1.0 0.0 0:02.08 top
31 root 25 5 0 0 0 S 0.3 0.0 2:53.77 ksmd
3676 root 20 0 0 0 0 S 0.3 0.0 0:00.06 kworker/u16:0
1 root 20 0 38148 6232 4032 S 0.0 0.1 0:03.71 systemd
此时使用 perf kvm --host stat live 可以看到几乎很少出现IO_INSTRUCTION
19:24:22.553073
Analyze events for all VMs, all VCPUs:
VM-EXIT Samples Samples% Time% Min Time Max Time Avg time
MSR_READ 1501 75.77% 0.44% 3.25us 23.38us 5.36us ( +- 0.75% )
MSR_WRITE 192 9.69% 0.16% 5.48us 49.34us 15.46us ( +- 3.90% )
HLT 174 8.78% 99.27% 4.16us 15716.32us 10431.50us ( +- 4.98% )
IO_INSTRUCTION 50 2.52% 0.09% 7.80us 83.96us 33.46us ( +- 6.40% )
EXTERNAL_INTERRUPT 39 1.97% 0.01% 3.47us 17.31us 6.20us ( +- 7.28% )
PENDING_INTERRUPT 7 0.35% 0.00% 4.97us 8.10us 6.39us ( +- 6.23% )
TPR_BELOW_THRESHOLD 7 0.35% 0.00% 7.16us 10.22us 8.94us ( +- 5.35% )
EPT_MISCONFIG 7 0.35% 0.01% 36.36us 42.87us 37.42us ( +- 2.43% )
PAUSE_INSTRUCTION 3 0.15% 0.00% 3.39us 3.72us 3.59us ( +- 2.83% )
EPT_VIOLATION 1 0.05% 0.00% 68.73us 68.73us 68.73us ( +- 0.00% )
Total Samples:1981, Total events handled time:1828459.05us.
此时NC上观察Windows虚拟机已经很少有SYS占用情况，响应也恢复正常。