Re: [PATCH v3 1/2] KVM: LAPIC: Optimize timer latency consider world switch time

[Sean Christopherson]

On Wed, Jun 12, 2019 at 05:40:18PM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> 
> Advance lapic timer tries to hidden the hypervisor overhead between the
> host emulated timer fires and the guest awares the timer is fired. However,
> even though after more sustaining optimizations, kvm-unit-tests/tscdeadline_latency 
> still awares ~1000 cycles latency since we lost the time between the end of 
> wait_lapic_expire and the guest awares the timer is fired. There are 
> codes between the end of wait_lapic_expire and the world switch, furthermore, 
> the world switch itself also has overhead. Actually the guest_tsc is equal 
> to the target deadline time in wait_lapic_expire is too late, guest will
> aware the latency between the end of wait_lapic_expire() and after vmentry 
> to the guest. This patch takes this time into consideration. 
> 
> The vmentry+vmexit time which is measured by kvm-unit-tests/vmexit.falt is 
> 1800 cycles on my 2.5GHz Skylake server, the vmentry_advance_ns module 
> parameter default value is 300ns, it can be tuned/reworked in the further.
> This patch can reduce average cyclictest latency from 3us to 2us on Skylake 
> server. (guest w/ nohz=off, idle=poll, host w/ preemption_timer=N, the 
> cyclictest latency is not too sensitive when preemption_timer=Y for this 
> optimization in my testing).
> 
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> ---
> v2 -> v3:
>  * read-only module parameter
>  * get_vmentry_advance_cycles() not inline
> v1 -> v2:
>  * rename get_vmentry_advance_delta to get_vmentry_advance_cycles
>  * cache vmentry_advance_cycles by setting param bit 0 
>  * add param max limit 
> 
>  arch/x86/kvm/lapic.c   | 33 ++++++++++++++++++++++++++++++---
>  arch/x86/kvm/lapic.h   |  3 +++
>  arch/x86/kvm/vmx/vmx.c |  2 +-
>  arch/x86/kvm/x86.c     |  8 ++++++++
>  arch/x86/kvm/x86.h     |  2 ++
>  5 files changed, 44 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index fcf42a3..c6d76f9 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1531,6 +1531,33 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
>  	apic->lapic_timer.timer_advance_ns = timer_advance_ns;
>  }
>  
> +u64 compute_vmentry_advance_cycles(struct kvm_vcpu *vcpu)
> +{
> +	u64 cycles;
> +	struct kvm_lapic *apic = vcpu->arch.apic;
> +
> +	cycles = vmentry_advance_ns * vcpu->arch.virtual_tsc_khz;
> +	do_div(cycles, 1000000);
> +
> +	apic->lapic_timer.vmentry_advance_cycles = cycles;
> +
> +	return cycles;
> +}
> +
> +u64 get_vmentry_advance_cycles(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_lapic *apic = vcpu->arch.apic;
> +
> +	if (unlikely(!vmentry_advance_ns))
> +		return 0;
> +
> +	if (likely(apic->lapic_timer.vmentry_advance_cycles))
> +		return apic->lapic_timer.vmentry_advance_cycles;
> +
> +	return compute_vmentry_advance_cycles(vcpu);

If vmentry_advance_ns is read-only, then we don't need to be able to
compute lapic_timer.vmentry_advance_cycles on demand, e.g. it can be set
during kvm_create_lapic() and recomputed in kvm_set_tsc_khz().

Alternatively, it could be handled purely in kvm_set_tsc_khz() if the call
to kvm_create_lapic() were moved before kvm_set_tsc_khz().

> +}
> +EXPORT_SYMBOL_GPL(get_vmentry_advance_cycles);
> +
>  void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_lapic *apic = vcpu->arch.apic;
> @@ -1544,7 +1571,7 @@ void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
>  
>  	tsc_deadline = apic->lapic_timer.expired_tscdeadline;
>  	apic->lapic_timer.expired_tscdeadline = 0;
> -	guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
> +	guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc()) + get_vmentry_advance_cycles(vcpu);
>  	apic->lapic_timer.advance_expire_delta = guest_tsc - tsc_deadline;
>  
>  	if (guest_tsc < tsc_deadline)
> @@ -1572,7 +1599,7 @@ static void start_sw_tscdeadline(struct kvm_lapic *apic)
>  	local_irq_save(flags);
>  
>  	now = ktime_get();
> -	guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
> +	guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc()) + get_vmentry_advance_cycles(vcpu);
>  	ns = (tscdeadline - guest_tsc) * 1000000ULL;
>  	do_div(ns, this_tsc_khz);
> @@ -2329,7 +2356,7 @@ int kvm_create_lapic(struct kvm_vcpu *vcpu, int timer_advance_ns)
>  		apic->lapic_timer.timer_advance_ns = timer_advance_ns;
>  		apic->lapic_timer.timer_advance_adjust_done = true;
>  	}
> -
> +	apic->lapic_timer.vmentry_advance_cycles = 0;
>  
>  	/*
>  	 * APIC is created enabled. This will prevent kvm_lapic_set_base from
> diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
> index f974a3d..fb32e69 100644
> --- a/arch/x86/kvm/lapic.h
> +++ b/arch/x86/kvm/lapic.h
> @@ -33,6 +33,7 @@ struct kvm_timer {
>  	u64 expired_tscdeadline;
>  	u32 timer_advance_ns;
>  	s64 advance_expire_delta;
> +	u64 vmentry_advance_cycles;
>  	atomic_t pending;			/* accumulated triggered timers */
>  	bool hv_timer_in_use;
>  	bool timer_advance_adjust_done;
> @@ -221,6 +222,8 @@ static inline int kvm_lapic_latched_init(struct kvm_vcpu *vcpu)
>  bool kvm_apic_pending_eoi(struct kvm_vcpu *vcpu, int vector);
>  
>  void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu);
> +u64 compute_vmentry_advance_cycles(struct kvm_vcpu *vcpu);
> +u64 get_vmentry_advance_cycles(struct kvm_vcpu *vcpu);
>  
>  bool kvm_intr_is_single_vcpu_fast(struct kvm *kvm, struct kvm_lapic_irq *irq,
>  			struct kvm_vcpu **dest_vcpu);
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 0861c71..0751a44 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7041,7 +7041,7 @@ static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc,
>  
>  	vmx = to_vmx(vcpu);
>  	tscl = rdtsc();
> -	guest_tscl = kvm_read_l1_tsc(vcpu, tscl);
> +	guest_tscl = kvm_read_l1_tsc(vcpu, tscl) + get_vmentry_advance_cycles(vcpu);
>  	delta_tsc = max(guest_deadline_tsc, guest_tscl) - guest_tscl;
>  	lapic_timer_advance_cycles = nsec_to_cycles(vcpu,
>  						    ktimer->timer_advance_ns);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 553c292..4b983bb 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -145,6 +145,12 @@ module_param(tsc_tolerance_ppm, uint, S_IRUGO | S_IWUSR);
>  static int __read_mostly lapic_timer_advance_ns = -1;
>  module_param(lapic_timer_advance_ns, int, S_IRUGO | S_IWUSR);
>  
> +/*
> + * lapic timer vmentry advance (tscdeadline mode only) in nanoseconds.
> + */
> +u32 __read_mostly vmentry_advance_ns = 300;

Enabling this by default makes me nervous, e.g. nothing guarantees that
future versions of KVM and/or CPUs will continue to have 300ns of overhead
between wait_lapic_expire() and VM-Enter.

If we want it enabled by default so that it gets tested, the default
value should be extremely conservative, e.g. set the default to a small
percentage (25%?) of the latency of VM-Enter itself on modern CPUs,
VM-Enter latency being the min between VMLAUNCH and VMLOAD+VMRUN+VMSAVE.

> +module_param(vmentry_advance_ns, uint, S_IRUGO);
> +
>  static bool __read_mostly vector_hashing = true;
>  module_param(vector_hashing, bool, S_IRUGO);
>  
> @@ -1592,6 +1598,8 @@ static int kvm_set_tsc_khz(struct kvm_vcpu *vcpu, u32 user_tsc_khz)
>  	kvm_get_time_scale(user_tsc_khz * 1000LL, NSEC_PER_SEC,
>  			   &vcpu->arch.virtual_tsc_shift,
>  			   &vcpu->arch.virtual_tsc_mult);
> +	if (vcpu->arch.apic && user_tsc_khz != vcpu->arch.virtual_tsc_khz)
> +		compute_vmentry_advance_cycles(vcpu);
>  	vcpu->arch.virtual_tsc_khz = user_tsc_khz;
>  
>  	/*
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index a470ff0..2174355 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -294,6 +294,8 @@ extern u64 kvm_supported_xcr0(void);
>  
>  extern unsigned int min_timer_period_us;
>  
> +extern unsigned int vmentry_advance_ns;
> +
>  extern bool enable_vmware_backdoor;
>  
>  extern struct static_key kvm_no_apic_vcpu;
> -- 
> 2.7.4
>