Re: [PATCH v4 1/2] KVM: LAPIC: Optimize timer latency consider world switch time

[Wanpeng Li]

ping again,
On Fri, 21 Jun 2019 at 17:44, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
>
> ping,
> On Fri, 14 Jun 2019 at 09:15, Wanpeng Li <kernellwp@xxxxxxxxx> 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_advance_ns module parameter is conservative 25ns by default(thanks
> > to Radim's kvm-unit-tests/vmentry_latency.flat), it can be tuned/reworked in
> > the future.
> >
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> > Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> > ---
> > v3 -> v4:
> >  * default value is 25ns
> >  * compute vmentry_advance_cycles in kvm_set_tsc_khz() path
> > 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   | 21 ++++++++++++++++++---
> >  arch/x86/kvm/lapic.h   |  2 ++
> >  arch/x86/kvm/vmx/vmx.c |  3 ++-
> >  arch/x86/kvm/x86.c     | 12 ++++++++++--
> >  arch/x86/kvm/x86.h     |  2 ++
> >  5 files changed, 34 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index e82a18c..e92e4e5 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -1528,6 +1528,19 @@ 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;
> > +}
> > +
> >  void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
> >  {
> >         struct kvm_lapic *apic = vcpu->arch.apic;
> > @@ -1541,7 +1554,8 @@ 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()) +
> > +               apic->lapic_timer.vmentry_advance_cycles;
> >         apic->lapic_timer.advance_expire_delta = guest_tsc - tsc_deadline;
> >
> >         if (guest_tsc < tsc_deadline)
> > @@ -1569,7 +1583,8 @@ 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()) +
> > +               apic->lapic_timer.vmentry_advance_cycles;
> >
> >         ns = (tscdeadline - guest_tsc) * 1000000ULL;
> >         do_div(ns, this_tsc_khz);
> > @@ -2326,7 +2341,7 @@ 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 3674717..7c38950 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;
> > @@ -226,6 +227,7 @@ 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);
> >
> >  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 8fbea03..dc81c78 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -7064,7 +7064,8 @@ 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) +
> > +               vcpu->arch.apic->lapic_timer.vmentry_advance_cycles;
> >         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 0a05a4e..5e79b6c 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 = 25;
> > +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 (user_tsc_khz != vcpu->arch.virtual_tsc_khz)
> > +               compute_vmentry_advance_cycles(vcpu);
> >         vcpu->arch.virtual_tsc_khz = user_tsc_khz;
> >
> >         /*
> > @@ -9134,8 +9142,6 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
> >         }
> >         vcpu->arch.pio_data = page_address(page);
> >
> > -       kvm_set_tsc_khz(vcpu, max_tsc_khz);
> > -
> >         r = kvm_mmu_create(vcpu);
> >         if (r < 0)
> >                 goto fail_free_pio_data;
> > @@ -9148,6 +9154,8 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
> >         } else
> >                 static_key_slow_inc(&kvm_no_apic_vcpu);
> >
> > +       kvm_set_tsc_khz(vcpu, max_tsc_khz);
> > +
> >         vcpu->arch.mce_banks = kzalloc(KVM_MAX_MCE_BANKS * sizeof(u64) * 4,
> >                                        GFP_KERNEL_ACCOUNT);
> >         if (!vcpu->arch.mce_banks) {
> > diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> > index e08a128..9998989 100644
> > --- a/arch/x86/kvm/x86.h
> > +++ b/arch/x86/kvm/x86.h
> > @@ -299,6 +299,8 @@ 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
> >