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SubjectRe: [PATCH v3] KVM: LAPIC: Recalculate apic map in batch
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On 26/02/20 03:41, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
>
> 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>
> ---
> v2 -> v3:
> * move apic_map_dirty to kvm_arch
> * add the suggestions from Paolo
>
> v1 -> v2:
> * add apic_map_dirty to kvm_lapic
> * error condition in kvm_apic_set_state, do recalcuate unconditionally
>
> arch/x86/include/asm/kvm_host.h | 1 +
> arch/x86/kvm/lapic.c | 46 ++++++++++++++++++++++++++++++++---------
> arch/x86/kvm/lapic.h | 1 +
> arch/x86/kvm/x86.c | 1 +
> 4 files changed, 39 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 40a0c0f..4380ed1 100644
> --- 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 afcd30d..de832aa 100644
> --- 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 @@ static void recalculate_apic_map(struct kvm *kvm)
> 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)
> @@ -1912,7 +1932,7 @@ 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;
> @@ -2018,6 +2038,8 @@ 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);
> @@ -2166,7 +2188,7 @@ 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;
> }
> }
>
> @@ -2207,6 +2229,7 @@ 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);
>
> @@ -2258,6 +2281,8 @@ 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);
> }
>
> /*
> @@ -2479,17 +2504,18 @@ 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
> --- 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 79bc995..d3802a2 100644
> --- 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);
>

Queued, thanks.

Paolo

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