linux-kernel - Re: [PATCH v4 2/2] KVM: nVMX: Fix preemption timer bit set in vmcs02 even if L1 doesn't enable it

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Date:	Thu, 7 Jul 2016 14:29:33 +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>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Haozhong Zhang <haozhong.zhang@...el.com>
Subject: Re: [PATCH v4 2/2] KVM: nVMX: Fix preemption timer bit set in vmcs02
 even if L1 doesn't enable it



On 07/07/2016 14:18, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> We will go to vcpu_run() loop after L0 emulates VMRESUME which maybe 
> incur kvm_sched_out and kvm_sched_in operations since cond_resched() 
> will be called once need resched. Preemption timer will be reprogrammed 
> if vCPU is scheduled to a different pCPU. Then the preemption timer 
> bit of vmcs02 will be set if L0 enable preemption timer to run L1 even 
> if L1 doesn't enable preemption timer to run L2.
> 
> This patch fix it by don't reprogram preemption timer of vmcs02 if L1's 
> vCPU is scheduled on diffent pCPU when we are in the way to vmresume 
> nested guest, and fallback to hrtimer based emulated method.
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Yunhong Jiang <yunhong.jiang@...el.com>
> Cc: Jan Kiszka <jan.kiszka@...mens.com>
> Cc: Haozhong Zhang <haozhong.zhang@...el.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
> v3 -> v4:
>  * fallback to hrtimer based emulated method when in the way to vmresume nested guest 
> 
>  arch/x86/kvm/x86.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 0cc6cf8..05137c0 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2743,8 +2743,9 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  			mark_tsc_unstable("KVM discovered backwards TSC");
>  
>  		if (kvm_lapic_hv_timer_in_use(vcpu) &&
> +			(is_guest_mode(vcpu) ||
>  				kvm_x86_ops->set_hv_timer(vcpu,
> -					kvm_get_lapic_tscdeadline_msr(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,
> 

Thanks, this is good as a fallback.  I'll try to fix it by getting the
pin-based execution controls right but if I fail this patch is okay.

Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>