preemempt timer exit

/* VMCS Encodings */
enum vmcs_field {
  // ...
	VMX_PREEMPTION_TIMER_VALUE      = 0x0000482E,

EXIT_REASON_PREEMPTION_TIMER

   0.44%  reason PREEMPTION_TIMER rip 0x433bb9 info 0 0
   0.24%  reason PREEMPTION_TIMER rip 0x7f496d8c2ffe info 0 0
   0.16%  reason PREEMPTION_TIMER rip 0x625722 info 0 0
   0.14%  reason PREEMPTION_TIMER rip 0x4339a6 info 0 0
   0.14%  reason PREEMPTION_TIMER rip 0x416448 info 0 0
   0.11%  reason PREEMPTION_TIMER rip 0x7f496d8c2eb0 info 0 0
   0.11%  reason PREEMPTION_TIMER rip 0x7f496d8c2ea4 info 0 0
   0.10%  reason PREEMPTION_TIMER rip 0x7f496d8c2c4a info 0 0

1. l1 虚拟机走 : handle_fastpath_preemption_timer
2. l2 虚拟机退出的话走 : handle_preemption_timer

steal time

• kvm_arch_vcpu_put
  • kvm_steal_time_set_preempted
    • vcpu->stat.preemption_reported++;
• /sys/kernel/debug/kvm/preemption_other
• /sys/kernel/debug/kvm/preemption_reported

  [root@bogon 93070-19]# cat preemption_other
  316
  [root@bogon 93070-19]# cat preemption_reported
  116
  

两个字段都是在 kvm_steal_time_set_preempted 中的，所以这个的含义是什么:

	/*
	 * The vCPU can be marked preempted if and only if the VM-Exit was on
	 * an instruction boundary and will not trigger guest emulation of any
	 * kind (see vcpu_run).  Vendor specific code controls (conservatively)
	 * when this is true, for example allowing the vCPU to be marked
	 * preempted if and only if the VM-Exit was due to a host interrupt.
	 */
	if (!vcpu->arch.at_instruction_boundary) {
		vcpu->stat.preemption_other++;
		return;
	}
  vcpu->stat.preemption_reported++;

其他

1. kvm_intel 通过 preemption_timer

   /* Guest_tsc -> host_tsc conversion requires 64-bit division.  */
   static int __read_mostly cpu_preemption_timer_multi;
   static bool __read_mostly enable_preemption_timer = 1;
   #ifdef CONFIG_X86_64
   module_param_named(preemption_timer, enable_preemption_timer, bool, S_IRUGO);
   #endif
   

2. 如果系统不忙的时候，可以做到没有任何 vmexit ，preemempt timer 完全不启用

问题

1. 嵌套虚拟化 vmx_start_preemption_timer
2. arm 是如何实现的，arm 环境中有 preemption_timer 吗?

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