linux-kernel - Re: [PATCH 1/2] KVM: lapic: fix preemption timer backward when TSC backward

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Date:   Fri, 2 Sep 2016 12:48:23 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Wanpeng Li <wanpeng.li@...mail.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Yunhong Jiang <yunhong.jiang@...el.com>
Subject: Re: [PATCH 1/2] KVM: lapic: fix preemption timer backward when TSC
 backward



On 30/08/2016 10:14, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> TSC_OFFSET will be adjusted if discovers TSC backward during vCPU load. 
> The preemption timer which will leverage guest tsc to reprogram its 
> preemption timer value is also reprogrammed if vCPU is scheded in to 
> a different pCPU. However, current implementation reprogram preemption 
> timer before TSC_OFFSET is adjusted to the right value, this will result 
> in preemption timer also backward and fire prematurity.
> 
> This patch fix it by adjusting TSC_OFFSET before reprogramming preemption 
> timer if TSC backward.
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Yunhong Jiang <yunhong.jiang@...el.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
>  arch/x86/kvm/x86.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 19f9f9e..699f872 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2743,16 +2743,16 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  		if (tsc_delta < 0)
>  			mark_tsc_unstable("KVM discovered backwards TSC");
>  
> -		if (kvm_lapic_hv_timer_in_use(vcpu) &&
> -				kvm_x86_ops->set_hv_timer(vcpu,
> -					kvm_get_lapic_tscdeadline_msr(vcpu)))
> -			kvm_lapic_switch_to_sw_timer(vcpu);
>  		if (check_tsc_unstable()) {
>  			u64 offset = kvm_compute_tsc_offset(vcpu,
>  						vcpu->arch.last_guest_tsc);
>  			kvm_x86_ops->write_tsc_offset(vcpu, offset);
>  			vcpu->arch.tsc_catchup = 1;
>  		}
> +		if (kvm_lapic_hv_timer_in_use(vcpu) &&
> +				kvm_x86_ops->set_hv_timer(vcpu,
> +					kvm_get_lapic_tscdeadline_msr(vcpu)))
> +			kvm_lapic_switch_to_sw_timer(vcpu);
>  		/*
>  		 * On a host with synchronized TSC, there is no need to update
>  		 * kvmclock on vcpu->cpu migration
> 

Queued for 4.8, thanks.

Paolo