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>
> ---
>  arch/x86/kvm/lapic.c | 97 ++++++++++++++++++++++++++++++++++++++++++++++++++--
>  arch/x86/kvm/lapic.h | 11 ++++++
>  arch/x86/kvm/trace.h | 22 ++++++++++++
>  arch/x86/kvm/vmx.c   |  8 +++++
>  arch/x86/kvm/x86.c   |  4 +++
>  5 files changed, 139 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> @@ -1343,6 +1343,85 @@ static void start_sw_tscdeadline(struct kvm_lapic *apic)
> +void kvm_lapic_arm_hv_timer(struct kvm_vcpu *vcpu)
> +{

Preemption timer needs to be adjusted on every vmentry while hrtimer is
set only once.  After how many vmentries does preemption timer surpass
the overhead of hrtimer?

> +	struct kvm_lapic *apic = vcpu->arch.apic;
> +	u64 tscdeadline, guest_tsc;
> +
> +	if (apic->lapic_timer.hv_timer_state == HV_TIMER_NOT_USED)
> +		return;
> +
> +	tscdeadline = apic->lapic_timer.tscdeadline;
> +	guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
> +
> +	if (tscdeadline >= guest_tsc)
> +		kvm_x86_ops->set_hv_timer(vcpu, tscdeadline - guest_tsc);
> +	else
> +		kvm_x86_ops->set_hv_timer(vcpu, 0);

Calling kvm_lapic_expired_hv_timer will avoid the useless immediate
vmexit after a vmentry.

Thanks.