On Fri, May 31, 2019 at 02:40:13PM +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_lapic_timer_advance_ns module parameter should be well tuned by > host admin, setting bit 0 to 1 to finally cache parameter in KVM. 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), kvm-unit-tests/tscdeadline_latency can reach 0. > > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> > Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx> > Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> > Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx> > --- > NOTE: rebase on https://lkml.org/lkml/2019/5/20/449 > 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 | 38 +++++++++++++++++++++++++++++++++++--- > arch/x86/kvm/lapic.h | 3 +++ > arch/x86/kvm/vmx/vmx.c | 2 +- > arch/x86/kvm/x86.c | 9 +++++++++ > arch/x86/kvm/x86.h | 2 ++ > 5 files changed, 50 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c > index fcf42a3..60587b5 100644 > --- a/arch/x86/kvm/lapic.c > +++ b/arch/x86/kvm/lapic.c > @@ -1531,6 +1531,38 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu, > apic->lapic_timer.timer_advance_ns = timer_advance_ns; > } > > +#define MAX_VMENTRY_ADVANCE_NS 1000 > + > +u64 compute_vmentry_advance_cycles(struct kvm_vcpu *vcpu) This can be static, unless get_vmentry_advance_cycles() is moved to lapic.h, in which case compute_vmentry_advance_cycles() would need to be exported. > +{ > + u64 cycles; > + struct kvm_lapic *apic = vcpu->arch.apic; > + u64 val = min_t(u32, vmentry_lapic_timer_advance_ns, MAX_VMENTRY_ADVANCE_NS); > + > + cycles = (val & ~1ULL) * vcpu->arch.virtual_tsc_khz; > + do_div(cycles, 1000000); > + > + /* setting bit 0 locks the value, it is cached */ > + if (val & 1) > + apic->lapic_timer.vmentry_advance_cycles = cycles; > + > + return cycles; > +} > + > +inline u64 get_vmentry_advance_cycles(struct kvm_vcpu *vcpu) This shouldn't be 'inline' since it's exported from a C file. That being said, I think it's short enough to define as a 'static inline' in lapic.h. > +{ > + struct kvm_lapic *apic = vcpu->arch.apic; > + > + if (!vmentry_lapic_timer_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); > +} > +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 +1576,7 @@ 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 +1604,7 @@ 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 +2361,7 @@ 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..70854a9 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); > +inline 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 a341663..255b5d5 100644 > --- a/arch/x86/kvm/vmx/vmx.c > +++ b/arch/x86/kvm/vmx/vmx.c > @@ -7047,7 +7047,7 @@ 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 69c3672e..0d4eb27 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -145,6 +145,13 @@ 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. Setting > + * bit 0 to 1 after well manually tuning to cache vmentry advance time. > + */ > +u32 __read_mostly vmentry_lapic_timer_advance_ns = 0; > +module_param(vmentry_lapic_timer_advance_ns, uint, S_IRUGO | S_IWUSR); > + > static bool __read_mostly vector_hashing = true; > module_param(vector_hashing, bool, S_IRUGO); > > @@ -1592,6 +1599,8 @@ 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 275b3b6..b0a3b84 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_lapic_timer_advance_ns; > + > extern bool enable_vmware_backdoor; > > extern struct static_key kvm_no_apic_vcpu; > -- > 2.7.4 >
