Re: [PATCHv2] KVM: optimize apic interrupt delivery

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Sep 11, 2012 at 05:10:23PM +0300, Michael S. Tsirkin wrote:
> On Tue, Sep 11, 2012 at 04:02:25PM +0300, Gleb Natapov wrote:
> > Most interrupt are delivered to only one vcpu. Use pre-build tables to
> > find interrupt destination instead of looping through all vcpus. In case
> > of logical mode loop only through vcpus in a logical cluster irq is sent
> > to.
> > 
> > Signed-off-by: Gleb Natapov <gleb@redhat.com>
> 
> Added Paul just to make sure we are using RCU correctly.
> Paul could you pls answer the question below?
> 
> > ---
> >  Changelog:
> > 
> >   - v1->v2
> >    * fix race Avi noticed
> >    * rcu_read_lock() out of the block as per Avi
> >    * fix rcu issues pointed to by MST. All but one. Still use
> >      call_rcu(). Do not think this is serious issue. If it is should be
> >      solved by RCU subsystem.
> >    * Fix phys_map overflow pointed to by MST
> >    * recalculate_apic_map() does not return error any more.
> >    * add optimization for low prio logical mode with one cpu as dst (it
> >      happens)
> > 
> > I did not rewrote kvm_irq_delivery_to_apic_fast() MST way since my way
> > looks cleaner to me.
> > 
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 64adb61..3ba8951 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -511,6 +511,14 @@ struct kvm_arch_memory_slot {
> >  	struct kvm_lpage_info *lpage_info[KVM_NR_PAGE_SIZES - 1];
> >  };
> >  
> > +struct kvm_apic_map {
> > +	struct rcu_head rcu;
> > +	bool flat;
> > +	u8 ldr_bits;
> > +	struct kvm_lapic *phys_map[256];
> > +	struct kvm_lapic *logical_map[16][16];
> > +};
> > +
> >  struct kvm_arch {
> >  	unsigned int n_used_mmu_pages;
> >  	unsigned int n_requested_mmu_pages;
> > @@ -528,6 +536,8 @@ struct kvm_arch {
> >  	struct kvm_ioapic *vioapic;
> >  	struct kvm_pit *vpit;
> >  	int vapics_in_nmi_mode;
> > +	struct kvm_apic_map *apic_map;
> > +	struct mutex apic_map_lock;
> >  
> >  	unsigned int tss_addr;
> >  	struct page *apic_access_page;
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 07ad628..06672e8 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -139,11 +139,92 @@ static inline int apic_enabled(struct kvm_lapic *apic)
> >  	(LVT_MASK | APIC_MODE_MASK | APIC_INPUT_POLARITY | \
> >  	 APIC_LVT_REMOTE_IRR | APIC_LVT_LEVEL_TRIGGER)
> >  
> > +static inline int apic_x2apic_mode(struct kvm_lapic *apic)
> > +{
> > +	return apic->vcpu->arch.apic_base & X2APIC_ENABLE;
> > +}
> > +
> >  static inline int kvm_apic_id(struct kvm_lapic *apic)
> >  {
> >  	return (kvm_apic_get_reg(apic, APIC_ID) >> 24) & 0xff;
> >  }
> >  
> > +static void kvm_apic_get_logical_id(u32 ldr, bool flat, u8 ldr_bits,
> > +		u16 *cid, u16 *lid)
> > +{
> > +	if (ldr_bits == 32) {
> > +		*cid = ldr >> 16;
> > +		*lid = ldr & 0xffff;
> > +	} else {
> > +		ldr = GET_APIC_LOGICAL_ID(ldr);
> > +
> > +		if (flat) {
> > +			*cid = 0;
> > +			*lid = ldr;
> > +		} else {
> > +			*cid = ldr >> 4;
> > +			*lid = ldr & 0xf;
> > +		}
> > +	}
> > +}
> > +
> > +static inline void recalculate_apic_map(struct kvm *kvm)
> > +{
> > +	struct kvm_apic_map *new, *old = NULL;
> > +	struct kvm_vcpu *vcpu;
> > +	int i;
> > +
> > +	new = kzalloc(sizeof(struct kvm_apic_map), GFP_KERNEL);
> > +
> > +	mutex_lock(&kvm->arch.apic_map_lock);
> > +
> > +	if (!new)
> > +		goto out;
> > +
> > +	new->ldr_bits = 8;
> > +	new->flat = true;
> > +	kvm_for_each_vcpu(i, vcpu, kvm) {
> > +		u16 cid, lid;
> > +		struct kvm_lapic *apic = vcpu->arch.apic;
> > +
> > +		if (!kvm_apic_present(vcpu))
> > +			continue;
> > +
> > +		if (apic_x2apic_mode(apic)) {
> > +			new->ldr_bits = 32;
> > +			new->flat = false;
> > +		} else if (kvm_apic_sw_enabled(apic) && new->flat &&
> > +				kvm_apic_get_reg(apic, APIC_DFR) == APIC_DFR_CLUSTER)
> > +			new->flat = false;
> > +
> > +		new->phys_map[kvm_apic_id(apic)] = apic;
> > +		kvm_apic_get_logical_id(kvm_apic_get_reg(apic, APIC_LDR),
> > +				new->flat, new->ldr_bits, &cid, &lid);
> > +
> > +		if (lid)
> > +			new->logical_map[cid][ffs(lid) - 1] = apic;
> > +	}
> > +out:
> > +	old = kvm->arch.apic_map;
> > +	rcu_assign_pointer(kvm->arch.apic_map, new);
> > +	mutex_unlock(&kvm->arch.apic_map_lock);
> > +
> > +	if (old)
> > +		kfree_rcu(old, rcu);
> > +}
> 
> Paul, I'd like to check something with you here:
> this function can be triggered by userspace,
> any number of times; we allocate
> a 2K chunk of memory that is later freed by
> kfree_rcu.
> 
> Is there a risk of DOS if RCU is delayed while
> lots of memory is queued up in this way?
> If yes is this a generic problem with kfree_rcu
> that should be addressed in core kernel?

There is indeed a risk.  The kfree_rcu() implementation cannot really
decide what to do here, especially given that it is callable with irqs
disabled.

The usual approach is to keep a per-CPU counter and count it down from
some number for each kfree_rcu().  When it reaches zero, invoke
synchronize_rcu() as well as kfree_rcu(), and then reset it to the
"some number" mentioned above.

In theory, I could create an API that did this.  In practice, I have no
idea how to choose the number -- much depends on the size of the object
being freed, for example.

The RCU usage looks otherwise sane at first glance.

							Thanx, Paul

> Thanks!
> 
> > +
> > +static inline void kvm_apic_set_id(struct kvm_lapic *apic, u8 id)
> > +{
> > +	apic_set_reg(apic, APIC_ID, id << 24);
> > +	recalculate_apic_map(apic->vcpu->kvm);
> > +}
> > +
> > +static inline void kvm_apic_set_ldr(struct kvm_lapic *apic, u32 id)
> > +{
> > +	apic_set_reg(apic, APIC_LDR, id);
> > +	recalculate_apic_map(apic->vcpu->kvm);
> > +}
> > +
> >  static inline int apic_lvt_enabled(struct kvm_lapic *apic, int lvt_type)
> >  {
> >  	return !(kvm_apic_get_reg(apic, lvt_type) & APIC_LVT_MASKED);
> > @@ -193,11 +274,6 @@ void kvm_apic_set_version(struct kvm_vcpu *vcpu)
> >  	apic_set_reg(apic, APIC_LVR, v);
> >  }
> >  
> > -static inline int apic_x2apic_mode(struct kvm_lapic *apic)
> > -{
> > -	return apic->vcpu->arch.apic_base & X2APIC_ENABLE;
> > -}
> > -
> >  static const unsigned int apic_lvt_mask[APIC_LVT_NUM] = {
> >  	LVT_MASK ,      /* part LVTT mask, timer mode mask added at runtime */
> >  	LVT_MASK | APIC_MODE_MASK,	/* LVTTHMR */
> > @@ -477,6 +553,79 @@ int kvm_apic_match_dest(struct kvm_vcpu *vcpu, struct kvm_lapic *source,
> >  	return result;
> >  }
> >  
> > +bool kvm_irq_delivery_to_apic_fast(struct kvm *kvm, struct kvm_lapic *src,
> > +		struct kvm_lapic_irq *irq, int *r)
> > +{
> > +	struct kvm_apic_map *map;
> > +	unsigned long bitmap = 1;
> > +	struct kvm_lapic **dst;
> > +	int i;
> > +
> > +	*r = -1;
> > +
> > +	if (irq->shorthand == APIC_DEST_SELF) {
> > +		*r = kvm_apic_set_irq(src->vcpu, irq);
> > +		return true;
> > +	}
> > +
> > +	if (irq->shorthand)
> > +		return false;
> > +
> > +	rcu_read_lock();
> > +	map = rcu_dereference(kvm->arch.apic_map);
> > +
> > +	if (!map) {
> > +		rcu_read_unlock();
> > +		return false;
> > +	}
> > +
> > +	if (irq->dest_mode == 0) { /* physical mode */
> > +		if (irq->delivery_mode == APIC_DM_LOWEST ||
> > +				irq->dest_id == 0xff) {
> > +			rcu_read_unlock();
> > +			return false;
> > +		}
> > +		dst = &map->phys_map[irq->dest_id & 0xff];
> > +	} else {
> > +		u16 cid, lid;
> > +		u32 mda = irq->dest_id;
> > +
> > +		if (map->ldr_bits == 8)
> > +			mda <<= 24;
> > +
> > +		kvm_apic_get_logical_id(mda, map->flat, map->ldr_bits,
> > +				&cid, &lid);
> > +		dst = map->logical_map[cid];
> > +
> > +		bitmap = lid;
> > +		if (irq->delivery_mode == APIC_DM_LOWEST &&
> > +				hweight_long(bitmap) > 1) {
> > +			int l = -1;
> > +			for_each_set_bit(i, &bitmap, 16) {
> > +				if (!dst[i])
> > +					continue;
> > +				if (l < 0)
> > +					l = i;
> > +				else if (kvm_apic_compare_prio(dst[i]->vcpu, dst[l]->vcpu) < 0)
> > +					l = i;
> > +			}
> > +
> > +			bitmap = (l >= 0) ? 1 << l : 0;
> > +		}
> > +	}
> > +
> > +	for_each_set_bit(i, &bitmap, 16) {
> > +		if (!dst[i])
> > +			continue;
> > +		if (*r < 0)
> > +			*r = 0;
> > +		*r += kvm_apic_set_irq(dst[i]->vcpu, irq);
> > +	}
> > +
> > +	rcu_read_unlock();
> > +	return true;
> > +}
> > +
> >  /*
> >   * Add a pending IRQ into lapic.
> >   * Return 1 if successfully added and 0 if discarded.
> > @@ -880,7 +1029,7 @@ static int apic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
> >  	switch (reg) {
> >  	case APIC_ID:		/* Local APIC ID */
> >  		if (!apic_x2apic_mode(apic))
> > -			apic_set_reg(apic, APIC_ID, val);
> > +			kvm_apic_set_id(apic, val >> 24);
> >  		else
> >  			ret = 1;
> >  		break;
> > @@ -896,15 +1045,16 @@ static int apic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
> >  
> >  	case APIC_LDR:
> >  		if (!apic_x2apic_mode(apic))
> > -			apic_set_reg(apic, APIC_LDR, val & APIC_LDR_MASK);
> > +			kvm_apic_set_ldr(apic, val & APIC_LDR_MASK);
> >  		else
> >  			ret = 1;
> >  		break;
> >  
> >  	case APIC_DFR:
> > -		if (!apic_x2apic_mode(apic))
> > +		if (!apic_x2apic_mode(apic)) {
> >  			apic_set_reg(apic, APIC_DFR, val | 0x0FFFFFFF);
> > -		else
> > +			recalculate_apic_map(apic->vcpu->kvm);
> > +		} else
> >  			ret = 1;
> >  		break;
> >  
> > @@ -1135,6 +1285,7 @@ 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);
> >  	}
> >  
> >  	if (!kvm_vcpu_is_bsp(apic->vcpu))
> > @@ -1144,7 +1295,7 @@ void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value)
> >  	if (apic_x2apic_mode(apic)) {
> >  		u32 id = kvm_apic_id(apic);
> >  		u32 ldr = ((id & ~0xf) << 16) | (1 << (id & 0xf));
> > -		apic_set_reg(apic, APIC_LDR, ldr);
> > +		kvm_apic_set_ldr(apic, ldr);
> >  	}
> >  	apic->base_address = apic->vcpu->arch.apic_base &
> >  			     MSR_IA32_APICBASE_BASE;
> > @@ -1169,7 +1320,7 @@ void kvm_lapic_reset(struct kvm_vcpu *vcpu)
> >  	/* Stop the timer in case it's a reset to an active apic */
> >  	hrtimer_cancel(&apic->lapic_timer.timer);
> >  
> > -	apic_set_reg(apic, APIC_ID, vcpu->vcpu_id << 24);
> > +	kvm_apic_set_id(apic, (u8)vcpu->vcpu_id);
> >  	kvm_apic_set_version(apic->vcpu);
> >  
> >  	for (i = 0; i < APIC_LVT_NUM; i++)
> > @@ -1180,7 +1331,7 @@ void kvm_lapic_reset(struct kvm_vcpu *vcpu)
> >  	apic_set_reg(apic, APIC_DFR, 0xffffffffU);
> >  	apic_set_spiv(apic, 0xff);
> >  	apic_set_reg(apic, APIC_TASKPRI, 0);
> > -	apic_set_reg(apic, APIC_LDR, 0);
> > +	kvm_apic_set_ldr(apic, 0);
> >  	apic_set_reg(apic, APIC_ESR, 0);
> >  	apic_set_reg(apic, APIC_ICR, 0);
> >  	apic_set_reg(apic, APIC_ICR2, 0);
> > @@ -1398,6 +1549,8 @@ void kvm_apic_post_state_restore(struct kvm_vcpu *vcpu,
> >  	/* set SPIV separately to get count of SW disabled APICs right */
> >  	apic_set_spiv(apic, *((u32 *)(s->regs + APIC_SPIV)));
> >  	memcpy(vcpu->arch.apic->regs, s->regs, sizeof *s);
> > +	/* call kvm_apic_set_id() to put apic into apic_map */
> > +	kvm_apic_set_id(apic, kvm_apic_id(apic));
> >  	kvm_apic_set_version(vcpu);
> >  
> >  	apic_update_ppr(apic);
> > diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
> > index 615a8b0..e5ebf9f 100644
> > --- a/arch/x86/kvm/lapic.h
> > +++ b/arch/x86/kvm/lapic.h
> > @@ -52,6 +52,9 @@ int kvm_apic_match_logical_addr(struct kvm_lapic *apic, u8 mda);
> >  int kvm_apic_set_irq(struct kvm_vcpu *vcpu, struct kvm_lapic_irq *irq);
> >  int kvm_apic_local_deliver(struct kvm_lapic *apic, int lvt_type);
> >  
> > +bool kvm_irq_delivery_to_apic_fast(struct kvm *kvm, struct kvm_lapic *src,
> > +		struct kvm_lapic_irq *irq, int *r);
> > +
> >  u64 kvm_get_apic_base(struct kvm_vcpu *vcpu);
> >  void kvm_set_apic_base(struct kvm_vcpu *vcpu, u64 data);
> >  void kvm_apic_post_state_restore(struct kvm_vcpu *vcpu,
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index f91e2c9..db6db8f 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -6269,6 +6269,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
> >  	set_bit(KVM_USERSPACE_IRQ_SOURCE_ID, &kvm->arch.irq_sources_bitmap);
> >  
> >  	raw_spin_lock_init(&kvm->arch.tsc_write_lock);
> > +	mutex_init(&kvm->arch.apic_map_lock);
> >  
> >  	return 0;
> >  }
> > @@ -6319,6 +6320,7 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
> >  		put_page(kvm->arch.apic_access_page);
> >  	if (kvm->arch.ept_identity_pagetable)
> >  		put_page(kvm->arch.ept_identity_pagetable);
> > +	kfree(kvm->arch.apic_map);
> >  }
> >  
> >  void kvm_arch_free_memslot(struct kvm_memory_slot *free,
> > diff --git a/virt/kvm/irq_comm.c b/virt/kvm/irq_comm.c
> > index 7118be0..3ca89c4 100644
> > --- a/virt/kvm/irq_comm.c
> > +++ b/virt/kvm/irq_comm.c
> > @@ -68,8 +68,13 @@ int kvm_irq_delivery_to_apic(struct kvm *kvm, struct kvm_lapic *src,
> >  	struct kvm_vcpu *vcpu, *lowest = NULL;
> >  
> >  	if (irq->dest_mode == 0 && irq->dest_id == 0xff &&
> > -			kvm_is_dm_lowest_prio(irq))
> > +			kvm_is_dm_lowest_prio(irq)) {
> >  		printk(KERN_INFO "kvm: apic: phys broadcast and lowest prio\n");
> > +		irq->delivery_mode = APIC_DM_FIXED;
> > +	}
> > +
> > +	if (kvm_irq_delivery_to_apic_fast(kvm, src, irq, &r))
> > +		return r;
> >  
> >  	kvm_for_each_vcpu(i, vcpu, kvm) {
> >  		if (!kvm_apic_present(vcpu))
> > --
> > 			Gleb.
>