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SubjectRe: [PATCH v3 1/2] KVM: LAPIC: Optimize timer latency consider world switch time
On Wed, Jun 12, 2019 at 05:40:18PM +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,
> 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@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Sean Christopherson <sean.j.christopherson@intel.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
> 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
>

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