KVM: LAPIC: Recalculate apic map in batch

In the vCPU reset and set APIC_BASE MSR path, the apic map will be recalculated
several times, each time it will consume 10+ us observed by ftrace in my
non-overcommit environment since the expensive memory allocate/mutex/rcu etc
operations. This patch optimizes it by recaluating apic map in batch, I hope
this can benefit the serverless scenario which can frequently create/destroy
VMs.

Before patch:

kvm_lapic_reset  ~27us

After patch:

kvm_lapic_reset  ~14us

Observed by ftrace, improve ~48%.

Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 681e230..6422737 100644 (file)
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -920,6 +920,7 @@ struct kvm_arch {
        atomic_t vapics_in_nmi_mode;
        struct mutex apic_map_lock;
        struct kvm_apic_map *apic_map;
+       bool apic_map_dirty;
 
        bool apic_access_page_done;
        unsigned long apicv_inhibit_reasons;

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 18a42dc..41bd49e 100644 (file)
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -164,14 +164,28 @@ static void kvm_apic_map_free(struct rcu_head *rcu)
        kvfree(map);
 }
 
-static void recalculate_apic_map(struct kvm *kvm)
+void kvm_recalculate_apic_map(struct kvm *kvm)
 {
        struct kvm_apic_map *new, *old = NULL;
        struct kvm_vcpu *vcpu;
        int i;
        u32 max_id = 255; /* enough space for any xAPIC ID */
 
+       if (!kvm->arch.apic_map_dirty) {
+               /*
+                * Read kvm->arch.apic_map_dirty before
+                * kvm->arch.apic_map
+                */
+               smp_rmb();
+               return;
+       }
+
        mutex_lock(&kvm->arch.apic_map_lock);
+       if (!kvm->arch.apic_map_dirty) {
+               /* Someone else has updated the map. */
+               mutex_unlock(&kvm->arch.apic_map_lock);
+               return;
+       }
 
        kvm_for_each_vcpu(i, vcpu, kvm)
                if (kvm_apic_present(vcpu))
@@ -236,6 +250,12 @@ out:
        old = rcu_dereference_protected(kvm->arch.apic_map,
                        lockdep_is_held(&kvm->arch.apic_map_lock));
        rcu_assign_pointer(kvm->arch.apic_map, new);
+       /*
+        * Write kvm->arch.apic_map before
+        * clearing apic->apic_map_dirty
+        */
+       smp_wmb();
+       kvm->arch.apic_map_dirty = false;
        mutex_unlock(&kvm->arch.apic_map_lock);
 
        if (old)
@@ -257,20 +277,20 @@ static inline void apic_set_spiv(struct kvm_lapic *apic, u32 val)
                else
                        static_key_slow_inc(&apic_sw_disabled.key);
 
-               recalculate_apic_map(apic->vcpu->kvm);
+               apic->vcpu->kvm->arch.apic_map_dirty = true;
        }
 }
 
 static inline void kvm_apic_set_xapic_id(struct kvm_lapic *apic, u8 id)
 {
        kvm_lapic_set_reg(apic, APIC_ID, id << 24);
-       recalculate_apic_map(apic->vcpu->kvm);
+       apic->vcpu->kvm->arch.apic_map_dirty = true;
 }
 
 static inline void kvm_apic_set_ldr(struct kvm_lapic *apic, u32 id)
 {
        kvm_lapic_set_reg(apic, APIC_LDR, id);
-       recalculate_apic_map(apic->vcpu->kvm);
+       apic->vcpu->kvm->arch.apic_map_dirty = true;
 }
 
 static inline u32 kvm_apic_calc_x2apic_ldr(u32 id)
@@ -286,7 +306,7 @@ static inline void kvm_apic_set_x2apic_id(struct kvm_lapic *apic, u32 id)
 
        kvm_lapic_set_reg(apic, APIC_ID, id);
        kvm_lapic_set_reg(apic, APIC_LDR, ldr);
-       recalculate_apic_map(apic->vcpu->kvm);
+       apic->vcpu->kvm->arch.apic_map_dirty = true;
 }
 
 static inline int apic_lvt_enabled(struct kvm_lapic *apic, int lvt_type)
@@ -1906,7 +1926,7 @@ int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
        case APIC_DFR:
                if (!apic_x2apic_mode(apic)) {
                        kvm_lapic_set_reg(apic, APIC_DFR, val | 0x0FFFFFFF);
-                       recalculate_apic_map(apic->vcpu->kvm);
+                       apic->vcpu->kvm->arch.apic_map_dirty = true;
                } else
                        ret = 1;
                break;
@@ -2012,6 +2032,8 @@ int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
                break;
        }
 
+       kvm_recalculate_apic_map(apic->vcpu->kvm);
+
        return ret;
 }
 EXPORT_SYMBOL_GPL(kvm_lapic_reg_write);
@@ -2160,7 +2182,7 @@ void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value)
                        static_key_slow_dec_deferred(&apic_hw_disabled);
                } else {
                        static_key_slow_inc(&apic_hw_disabled.key);
-                       recalculate_apic_map(vcpu->kvm);
+                       vcpu->kvm->arch.apic_map_dirty = true;
                }
        }
 
@@ -2201,6 +2223,7 @@ void kvm_lapic_reset(struct kvm_vcpu *vcpu, bool init_event)
        if (!apic)
                return;
 
+       vcpu->kvm->arch.apic_map_dirty = false;
        /* Stop the timer in case it's a reset to an active apic */
        hrtimer_cancel(&apic->lapic_timer.timer);
 
@@ -2252,6 +2275,8 @@ void kvm_lapic_reset(struct kvm_vcpu *vcpu, bool init_event)
 
        vcpu->arch.apic_arb_prio = 0;
        vcpu->arch.apic_attention = 0;
+
+       kvm_recalculate_apic_map(vcpu->kvm);
 }
 
 /*
@@ -2473,17 +2498,18 @@ int kvm_apic_set_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s)
        struct kvm_lapic *apic = vcpu->arch.apic;
        int r;
 
-
        kvm_lapic_set_base(vcpu, vcpu->arch.apic_base);
        /* set SPIV separately to get count of SW disabled APICs right */
        apic_set_spiv(apic, *((u32 *)(s->regs + APIC_SPIV)));
 
        r = kvm_apic_state_fixup(vcpu, s, true);
-       if (r)
+       if (r) {
+               kvm_recalculate_apic_map(vcpu->kvm);
                return r;
+       }
        memcpy(vcpu->arch.apic->regs, s->regs, sizeof(*s));
 
-       recalculate_apic_map(vcpu->kvm);
+       kvm_recalculate_apic_map(vcpu->kvm);
        kvm_apic_set_version(vcpu);
 
        apic_update_ppr(apic);

diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index ec6fbfe..7581bc2 100644 (file)
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -78,6 +78,7 @@ void kvm_lapic_set_tpr(struct kvm_vcpu *vcpu, unsigned long cr8);
 void kvm_lapic_set_eoi(struct kvm_vcpu *vcpu);
 void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value);
 u64 kvm_lapic_get_base(struct kvm_vcpu *vcpu);
+void kvm_recalculate_apic_map(struct kvm *kvm);
 void kvm_apic_set_version(struct kvm_vcpu *vcpu);
 int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val);
 int kvm_lapic_reg_read(struct kvm_lapic *apic, u32 offset, int len,

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 864d0ad..c5762c0 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -350,6 +350,7 @@ int kvm_set_apic_base(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
        }
 
        kvm_lapic_set_base(vcpu, msr_info->data);
+       kvm_recalculate_apic_map(vcpu->kvm);
        return 0;
 }
 EXPORT_SYMBOL_GPL(kvm_set_apic_base);