Re: [RFC PATCH V2 4/4] Utilize the vmx preemption timer for tsc deadline timer

["Radim Krčmář" <rkrcmar@redhat.com>]

2016-05-24 15:27-0700, Yunhong Jiang:
> From: Yunhong Jiang <yunhong.jiang@gmail.com>
> 
> Utilizing the VMX preemption timer for tsc deadline timer
> virtualization. The VMX preemption timer is armed when the vCPU is
> running, and a VMExit will happen if the virtual TSC deadline timer
> expires.
> 
> When the vCPU thread is scheduled out, the tsc deadline timer
> virtualization will be switched to use the current solution, i.e. use
> the timer for it. It's switched back to VMX preemption timer when the
> vCPU thread is scheduled int.
> 
> This solution avoids the complex OS's hrtimer system, and also the host
> timer interrupt handling cost, with a preemption_timer VMexit. It fits
> well for some NFV usage scenario, when the vCPU is bound to a pCPU and
> the pCPU is isolated, or some similar scenario.
> 
> However, it possibly has impact if the vCPU thread is scheduled in/out
> very frequently, because it switches from/to the hrtimer emulation a lot.
> 
> Signed-off-by: Yunhong Jiang <yunhong.jiang@intel.com>
> ---
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>  static const int kvm_vmx_max_exit_handlers =
> @@ -8678,6 +8684,8 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)
>  		vmx_set_interrupt_shadow(vcpu, 0);
>  
> +	kvm_lapic_arm_hv_timer(vcpu);
> +

Every cycle that the host spends in root mode after setting the timeout
makes the preemtion timer delayed.  Can't we move the setup closer to
vmlaunch, i.e. after atomic_switch_perf_msrs()?

>  	if (vmx->guest_pkru_valid)
>  		__write_pkru(vmx->guest_pkru);
>