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SubjectRe: [PATCH v2 4/4] KVM: LAPIC: Optimize timer latency further
On Wed, May 15, 2019 at 12:11:54PM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
>
> Advance lapic timer tries to hidden the hypervisor overhead between the
> host emulated timer fires and the guest awares the timer is fired. However,
> it just hidden the time between apic_timer_fn/handle_preemption_timer ->
> wait_lapic_expire, instead of the real position of vmentry which is
> mentioned in the orignial commit d0659d946be0 ("KVM: x86: add option to
> advance tscdeadline hrtimer expiration"). There is 700+ cpu cycles between
> the end of wait_lapic_expire and before world switch on my haswell desktop,
> it will be 2400+ cycles if vmentry_l1d_flush is tuned to always.
>
> This patch tries to narrow the last gap(wait_lapic_expire -> world switch),
> it takes the real overhead time between apic_timer_fn/handle_preemption_timer
> and before world switch into consideration when adaptively tuning timer
> advancement. The patch can reduce 40% latency (~1600+ cycles to ~1000+ cycles
> on a haswell desktop) for kvm-unit-tests/tscdeadline_latency when testing
> busy waits.
>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Sean Christopherson <sean.j.christopherson@intel.com>
> Cc: Liran Alon <liran.alon@oracle.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
> arch/x86/kvm/lapic.c | 3 ++-
> arch/x86/kvm/lapic.h | 2 +-
> arch/x86/kvm/svm.c | 4 ++++
> arch/x86/kvm/vmx/vmx.c | 4 ++++
> arch/x86/kvm/x86.c | 3 ---
> 5 files changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index af38ece..63513de 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1531,7 +1531,7 @@ static inline void adaptive_tune_timer_advancement(struct kvm_vcpu *vcpu,
> apic->lapic_timer.timer_advance_ns = timer_advance_ns;
> }
>
> -void wait_lapic_expire(struct kvm_vcpu *vcpu)
> +void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
> {
> struct kvm_lapic *apic = vcpu->arch.apic;
> u64 guest_tsc, tsc_deadline;
> @@ -1553,6 +1553,7 @@ void wait_lapic_expire(struct kvm_vcpu *vcpu)
> if (unlikely(!apic->lapic_timer.timer_advance_adjust_done))
> adaptive_tune_timer_advancement(vcpu, apic->lapic_timer.advance_expire_delta);
> }
> +EXPORT_SYMBOL_GPL(kvm_wait_lapic_expire);
>
> static void start_sw_tscdeadline(struct kvm_lapic *apic)
> {
> diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
> index 3e72a25..f974a3d 100644
> --- a/arch/x86/kvm/lapic.h
> +++ b/arch/x86/kvm/lapic.h
> @@ -220,7 +220,7 @@ static inline int kvm_lapic_latched_init(struct kvm_vcpu *vcpu)
>
> bool kvm_apic_pending_eoi(struct kvm_vcpu *vcpu, int vector);
>
> -void wait_lapic_expire(struct kvm_vcpu *vcpu);
> +void kvm_wait_lapic_expire(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/svm.c b/arch/x86/kvm/svm.c
> index 406b558..740fb3f 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -5646,6 +5646,10 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
> */
> x86_spec_ctrl_set_guest(svm->spec_ctrl, svm->virt_spec_ctrl);
>
> + if (lapic_in_kernel(vcpu) &&
> + vcpu->arch.apic->lapic_timer.timer_advance_ns)
> + kvm_wait_lapic_expire(vcpu);
> +
> local_irq_enable();
>
> asm volatile (
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 9663d41..1c49946 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6437,6 +6437,10 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
> if (vcpu->arch.cr2 != read_cr2())
> write_cr2(vcpu->arch.cr2);
>
> + if (lapic_in_kernel(vcpu) &&
> + vcpu->arch.apic->lapic_timer.timer_advance_ns)
> + kvm_wait_lapic_expire(vcpu);

One potential hiccup with this approach is that we're now accessing more
data after flushing the L1. Not sure if that's actually a problem here,
but it probably should be explicitly addressed/considered.

> +
> vmx->fail = __vmx_vcpu_run(vmx, (unsigned long *)&vcpu->arch.regs,
> vmx->loaded_vmcs->launched);
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 1d89cb9..0eb9549 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7894,9 +7894,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> }
>
> trace_kvm_entry(vcpu->vcpu_id);
> - if (lapic_in_kernel(vcpu) &&
> - vcpu->arch.apic->lapic_timer.timer_advance_ns)
> - wait_lapic_expire(vcpu);
> guest_enter_irqoff();
>
> fpregs_assert_state_consistent();
> --
> 2.7.4
>

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