linux-kernel - Re: [PATCH] KVM: X86: Fix preempt the preemption timer cancel

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Date:   Fri, 26 May 2017 17:57:32 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH] KVM: X86: Fix preempt the preemption timer cancel



On 21/05/2017 05:32, Wanpeng Li wrote:
>           CPU0                              CPU1 
> 
> vmx_cancel_hv_timer
>   vCPU0's vmx->hv_deadline_tsc = -1
> 
>   preempt occur
> 
>                                      clear preemption timer field in CPU1's active vmcs
>                                      vCPU0's apic_timer.hv_timer_in_use = false
> vmx_vcpu_run(vCPU0)
>   vmx_arm_hv_timer
>     if (vmx->hv_deadline_tsc == -1)
> 	  nothing change
> 	 
> handle_preemption_timer(vCPU0)
>   kvm_lapic_expired_hv_timer
>     WARN_ON(!apic->lapic_timer.hv_timer_in_use); 


I think it's more like this, what do you think?

          CPU0                    CPU1

  preemption timer vmexit
  handle_preemption_timer(vCPU0)
    kvm_lapic_expired_hv_timer
      vmx_cancel_hv_timer
        vmx->hv_deadline_tsc = -1
        vmcs_clear_bits
        /* hv_timer_in_use still true */
  sched_out
                           sched_in
                           kvm_arch_vcpu_load
                             vmx_set_hv_timer
                               write vmx->hv_deadline_tsc
                               vmcs_set_bits
                           /* back in kvm_lapic_expired_hv_timer */
                           hv_timer_in_use = false
                           ...
                           vmx_vcpu_run
                             vmx_arm_hv_run
                               write preemption timer deadline
                             spurious preemption timer vmexit
                               handle_preemption_timer(vCPU0)
                                 kvm_lapic_expired_hv_timer
                                   WARN_ON(!apic->lapic_timer.hv_timer_in_use);


Thanks,

Paolo