Re: [PATCHv6 8/8] kvm: host side for eoi optimization

[Marcelo Tosatti <mtosatti@redhat.com>]

On Sun, Jun 03, 2012 at 10:28:43AM +0300, Michael S. Tsirkin wrote:
> Implementation of PV EOI using shared memory.
> This reduces the number of exits an interrupt
> causes as much as by half.
> 
> The idea is simple: there's a bit, per APIC, in guest memory,
> that tells the guest that it does not need EOI.
> We set it before injecting an interrupt and clear
> before injecting a nested one. Guest tests it using
> a test and clear operation - this is necessary
> so that host can detect interrupt nesting -
> and if set, it can skip the EOI MSR.
> 
> There's a new MSR to set the address of said register
> in guest memory. Otherwise not much changed:
> - Guest EOI is not required
> - Register is tested & ISR is automatically cleared on exit
> 
> For testing results see description of previous patch
> 'kvm_para: guest side for eoi avoidance'.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  arch/x86/include/asm/kvm_host.h |   12 ++++
>  arch/x86/kvm/cpuid.c            |    1 +
>  arch/x86/kvm/lapic.c            |  140 +++++++++++++++++++++++++++++++++++++-
>  arch/x86/kvm/lapic.h            |    2 +
>  arch/x86/kvm/trace.h            |   34 ++++++++++
>  arch/x86/kvm/x86.c              |    7 ++
>  6 files changed, 192 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index db7c1f2..dfc066c 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -175,6 +175,13 @@ enum {
>  
>  /* apic attention bits */
>  #define KVM_APIC_CHECK_VAPIC	0
> +/*
> + * The following bit is set with PV-EOI, unset on EOI.
> + * We detect PV-EOI changes by guest by comparing
> + * this bit with PV-EOI in guest memory.
> + * See the implementation in apic_update_pv_eoi.
> + */ 
> +#define KVM_APIC_PV_EOI_PENDING	1
>  
>  /*
>   * We don't want allocation failures within the mmu code, so we preallocate
> @@ -484,6 +491,11 @@ struct kvm_vcpu_arch {
>  		u64 length;
>  		u64 status;
>  	} osvw;
> +
> +	struct {
> +		u64 msr_val;
> +		struct gfn_to_hva_cache data;
> +	} pv_eoi;
>  };
>  
>  struct kvm_lpage_info {
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 7df1c6d..61ccbdf 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -409,6 +409,7 @@ static int do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>  			     (1 << KVM_FEATURE_NOP_IO_DELAY) |
>  			     (1 << KVM_FEATURE_CLOCKSOURCE2) |
>  			     (1 << KVM_FEATURE_ASYNC_PF) |
> +			     (1 << KVM_FEATURE_PV_EOI) |
>  			     (1 << KVM_FEATURE_CLOCKSOURCE_STABLE_BIT);
>  
>  		if (sched_info_on())
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index db54e63..d16cfc5 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -306,6 +306,54 @@ int kvm_apic_set_irq(struct kvm_vcpu *vcpu, struct kvm_lapic_irq *irq)
>  			irq->level, irq->trig_mode);
>  }
>  
> +static int pv_eoi_put_user(struct kvm_vcpu *vcpu, u8 val)
> +{
> +
> +	return kvm_write_guest_cached(vcpu->kvm, &vcpu->arch.pv_eoi.data, &val,
> +				      sizeof(val));
> +}
> +
> +static int pv_eoi_get_user(struct kvm_vcpu *vcpu, u8 *val)
> +{
> +
> +	return kvm_read_guest_cached(vcpu->kvm, &vcpu->arch.pv_eoi.data, val,
> +				      sizeof(*val));
> +}
> +
> +static inline bool pv_eoi_enabled(struct kvm_vcpu *vcpu)
> +{
> +	return vcpu->arch.pv_eoi.msr_val & KVM_MSR_ENABLED;
> +}
> +
> +static bool pv_eoi_get_pending(struct kvm_vcpu *vcpu)
> +{
> +	u8 val;
> +	if (pv_eoi_get_user(vcpu, &val) < 0)
> +		apic_debug("Can't read EOI MSR value: 0x%llx\n",
> +			   (unsigned long long)vcpi->arch.pv_eoi.msr_val);
> +	return val & 0x1;
> +}
> +
> +static void pv_eoi_set_pending(struct kvm_vcpu *vcpu)
> +{
> +	if (pv_eoi_put_user(vcpu, KVM_PV_EOI_ENABLED) < 0) {
> +		apic_debug("Can't set EOI MSR value: 0x%llx\n",
> +			   (unsigned long long)vcpi->arch.pv_eoi.msr_val);
> +		return;
> +	}
> +	__set_bit(KVM_APIC_PV_EOI_PENDING, &vcpu->arch.apic_attention);
> +}
> +
> +static void pv_eoi_clr_pending(struct kvm_vcpu *vcpu)
> +{
> +	if (pv_eoi_put_user(vcpu, KVM_PV_EOI_DISABLED) < 0) {
> +		apic_debug("Can't clear EOI MSR value: 0x%llx\n",
> +			   (unsigned long long)vcpi->arch.pv_eoi.msr_val);
> +		return;
> +	}
> +	__clear_bit(KVM_APIC_PV_EOI_PENDING, &vcpu->arch.apic_attention);
> +}
> +
>  static inline int apic_find_highest_isr(struct kvm_lapic *apic)
>  {
>  	int result;
> @@ -522,15 +570,18 @@ int kvm_apic_compare_prio(struct kvm_vcpu *vcpu1, struct kvm_vcpu *vcpu2)
>  	return vcpu1->arch.apic_arb_prio - vcpu2->arch.apic_arb_prio;
>  }
>  
> -static void apic_set_eoi(struct kvm_lapic *apic)
> +static int apic_set_eoi(struct kvm_lapic *apic)
>  {
>  	int vector = apic_find_highest_isr(apic);
> +
> +	trace_kvm_eoi(apic, vector);
> +
>  	/*
>  	 * Not every write EOI will has corresponding ISR,
>  	 * one example is when Kernel check timer on setup_IO_APIC
>  	 */
>  	if (vector == -1)
> -		return;
> +		return vector;
>  
>  	apic_clear_isr(vector, apic);
>  	apic_update_ppr(apic);
> @@ -545,6 +596,7 @@ static void apic_set_eoi(struct kvm_lapic *apic)
>  		kvm_ioapic_update_eoi(apic->vcpu->kvm, vector, trigger_mode);
>  	}
>  	kvm_make_request(KVM_REQ_EVENT, apic->vcpu);
> +	return vector;
>  }
>  
>  static void apic_send_ipi(struct kvm_lapic *apic)
> @@ -1127,6 +1179,7 @@ void kvm_lapic_reset(struct kvm_vcpu *vcpu)
>  	atomic_set(&apic->lapic_timer.pending, 0);
>  	if (kvm_vcpu_is_bsp(vcpu))
>  		vcpu->arch.apic_base |= MSR_IA32_APICBASE_BSP;
> +	vcpu->arch.pv_eoi.msr_val = 0;
>  	apic_update_ppr(apic);
>  
>  	vcpu->arch.apic_arb_prio = 0;
> @@ -1327,11 +1380,51 @@ void __kvm_migrate_apic_timer(struct kvm_vcpu *vcpu)
>  		hrtimer_start_expires(timer, HRTIMER_MODE_ABS);
>  }
>  
> +/*
> + * apic_sync_pv_eoi_from_guest - called on vmexit or cancel interrupt
> + *
> + * Detect whether guest triggered PV EOI since the
> + * last entry. If yes, set EOI on guests's behalf.
> + * Clear PV EOI in guest memory in any case.
> + */
> +static void apic_sync_pv_eoi_from_guest(struct kvm_vcpu *vcpu,
> +					struct kvm_lapic *apic)
> +{
> +	bool pending;
> +	int vector;
> +	/*
> +	 * PV EOI state is derived from KVM_APIC_PV_EOI_PENDING in host
> +	 * and KVM_PV_EOI_ENABLED in guest memory as follows:
> +	 *
> +	 * KVM_APIC_PV_EOI_PENDING is unset:
> +	 * 	-> host disabled PV EOI.
> +	 * KVM_APIC_PV_EOI_PENDING is set, KVM_PV_EOI_ENABLED is set:
> +	 * 	-> host enabled PV EOI, guest did not execute EOI yet.
> +	 * KVM_APIC_PV_EOI_PENDING is set, KVM_PV_EOI_ENABLED is unset:
> +	 * 	-> host enabled PV EOI, guest executed EOI.
> +	 */
> +	BUG_ON(!pv_eoi_enabled(vcpu));
> +	pending = pv_eoi_get_pending(vcpu);
> +	/*
> +	 * Clear pending bit in any case: it will be set again on vmentry.
> +	 * While this might not be ideal from performance point of view,
> +	 * this makes sure pv eoi is only enabled when we know it's safe.
> +	 */
> +	pv_eoi_clr_pending(vcpu);
> +	if (pending)
> +		return;
> +	vector = apic_set_eoi(apic);
> +	trace_kvm_pv_eoi(apic, vector);
> +}
> +
>  void kvm_lapic_sync_from_vapic(struct kvm_vcpu *vcpu)
>  {
>  	u32 data;
>  	void *vapic;
>  
> +	if (test_bit(KVM_APIC_PV_EOI_PENDING, &vcpu->arch.apic_attention))
> +		apic_sync_pv_eoi_from_guest(vcpu, vcpu->arch.apic);
> +
>  	if (!test_bit(KVM_APIC_CHECK_VAPIC, &vcpu->arch.apic_attention))
>  		return;
>  
> @@ -1342,17 +1435,44 @@ void kvm_lapic_sync_from_vapic(struct kvm_vcpu *vcpu)
>  	apic_set_tpr(vcpu->arch.apic, data & 0xff);
>  }
>  
> +/*
> + * apic_sync_pv_eoi_to_guest - called before vmentry
> + *
> + * Detect whether it's safe to enable PV EOI and
> + * if yes do so.
> + */
> +static void apic_sync_pv_eoi_to_guest(struct kvm_vcpu *vcpu,
> +					struct kvm_lapic *apic)
> +{
> +	if (!pv_eoi_enabled(vcpu) ||
> +	    /* IRR set or many bits in ISR: could be nested. */
> +	    unlikely(apic->irr_pending) ||
> +	    /* Cache not set: could be safe but we don't bother. */
> +	    unlikely(apic->isr_cache == -1) ||
> +	    /* Need EOI to update ioapic. */
> +	    unlikely(kvm_ioapic_handles_vector(vcpu->kvm, apic->isr_cache))) {

This adds an lfence. Can't you move this logic to ioapic configuration
time?

Otherwise looks good (clear and very well commented).