Timer delays in VM

From: Eugene Huang <eugeneh@nvidia.com>
To: "kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>
Subject: Timer delays in VM
Date: Mon, 28 Feb 2022 18:02:50 +0000	[thread overview]
Message-ID: <BYAPR12MB31927AEB9D0A4068ED12826DD9019@BYAPR12MB3192.namprd12.prod.outlook.com> (raw)

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Hi,

I am running qemu on an arm64 CentOS host. Inside a ubuntu VM, a process runs a timer created using timer_t:
ev.sigev_notify_function = m_callback;
...
timer_create(CLOCK_MONOTONIC, &ev, &m_timer_t);

This timer sometimes has significant delays. For example, the 50 ms timer can have a callback delay of 100ms.

I did a host kernel trace and see a lot of WFx kvm_exits, and  the following events between kvm_exit and kvm_entry:
kvm_exit
kvm_wfx_arm64
kvm_get_timer_map
sched_switch
kvm_timer_save_state
kvm_timer_update_irq
vgic_update_irq_pending
kvm_timer_restore_state
kvm_vcpu_wakeup
kvm_arm_setup_debug
kvm_arm_set_dreg32
kvm_entry

I have the following questions:

  1.  Why there are a lot WFx exits? Is the timer dependent on it?
  2.  Does this timer rely on kvm timer irq injection?
  3.  What can be any possible causes for the timer delay? Are there some locking mechanisms which can cause the delay?
  4.  What parameters can tune this timer?

Thanks.

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