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

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Message-ID: <9c21e92e-0754-7eda-76bc-2418abe8cd5b@redhat.com>
Date:   Fri, 26 May 2017 17:45:30 +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:
> 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); 
>   
> Preemption can occur during cancel preemption timer, and there will be inconsistent 
> status in lapic, vmx and vmcs field. This patch fixes it by disable preemption for 
> cancelling preemption timer.

I see, so the purpose is to serialize against kvm_arch_vcpu_load.  Nice
catch, I've queued the patch for kvm/master.

Paolo

> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
>  arch/x86/kvm/lapic.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index c329d28..6e6f345 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1495,8 +1495,10 @@ EXPORT_SYMBOL_GPL(kvm_lapic_hv_timer_in_use);
>  
>  static void cancel_hv_timer(struct kvm_lapic *apic)
>  {
> +	preempt_disable();
>  	kvm_x86_ops->cancel_hv_timer(apic->vcpu);
>  	apic->lapic_timer.hv_timer_in_use = false;
> +	preempt_enable();
>  }
>  
>  static bool start_hv_timer(struct kvm_lapic *apic)
> --