Re: [PATCH 08/10] Move IO APIC to its own lock.

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Jul 14, 2009 at 05:30:43PM +0300, Gleb Natapov wrote:
> 
> Signed-off-by: Gleb Natapov <gleb@redhat.com>
> ---
>  arch/ia64/kvm/kvm-ia64.c |   27 ++++++++++++++++++------
>  arch/x86/kvm/lapic.c     |    5 +---
>  arch/x86/kvm/x86.c       |   30 ++++++++++++++++++---------
>  virt/kvm/ioapic.c        |   50 ++++++++++++++++++++++++++++-----------------
>  virt/kvm/ioapic.h        |    1 +
>  5 files changed, 73 insertions(+), 40 deletions(-)
> 
> diff --git a/arch/ia64/kvm/kvm-ia64.c b/arch/ia64/kvm/kvm-ia64.c
> index 0ad09f0..dd7ef2d 100644
> --- a/arch/ia64/kvm/kvm-ia64.c
> +++ b/arch/ia64/kvm/kvm-ia64.c
> @@ -850,9 +850,16 @@ static int kvm_vm_ioctl_get_irqchip(struct kvm *kvm,
>  
>  	r = 0;
>  	switch (chip->chip_id) {
> -	case KVM_IRQCHIP_IOAPIC:
> -		memcpy(&chip->chip.ioapic, ioapic_irqchip(kvm),
> -				sizeof(struct kvm_ioapic_state));
> +	case KVM_IRQCHIP_IOAPIC: {
> +		struct kvm_ioapic *ioapic = ioapic_irqchip(kvm);
> +		if (ioapic) {
> +			spin_lock(&ioapic->lock);
> +			memcpy(&chip->chip.ioapic, ioapic,
> +			       sizeof(struct kvm_ioapic_state));
> +			spin_unlock(&ioapic->lock);
> +		} else
> +			r = -EINVAL;
> +	}
>  		break;
>  	default:
>  		r = -EINVAL;
> @@ -867,10 +874,16 @@ static int kvm_vm_ioctl_set_irqchip(struct kvm *kvm, struct kvm_irqchip *chip)
>  
>  	r = 0;
>  	switch (chip->chip_id) {
> -	case KVM_IRQCHIP_IOAPIC:
> -		memcpy(ioapic_irqchip(kvm),
> -				&chip->chip.ioapic,
> -				sizeof(struct kvm_ioapic_state));
> +	case KVM_IRQCHIP_IOAPIC: {
> +		struct kvm_ioapic *ioapic = ioapic_irqchip(kvm);
> +		if (ioapic) {
> +			spin_lock(&ioapic->lock);
> +			memcpy(ioapic, &chip->chip.ioapic,
> +			       sizeof(struct kvm_ioapic_state));
> +			spin_unlock(&ioapic->lock);
> +		} else
> +			r = -EINVAL;
> +	}
>  		break;
>  	default:
>  		r = -EINVAL;
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index e2e2849..61ffcfc 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -471,11 +471,8 @@ static void apic_set_eoi(struct kvm_lapic *apic)
>  		trigger_mode = IOAPIC_LEVEL_TRIG;
>  	else
>  		trigger_mode = IOAPIC_EDGE_TRIG;
> -	if (!(apic_get_reg(apic, APIC_SPIV) & APIC_SPIV_DIRECTED_EOI)) {
> -		mutex_lock(&apic->vcpu->kvm->irq_lock);
> +	if (!(apic_get_reg(apic, APIC_SPIV) & APIC_SPIV_DIRECTED_EOI))
>  		kvm_ioapic_update_eoi(apic->vcpu->kvm, vector, trigger_mode);
> -		mutex_unlock(&apic->vcpu->kvm->irq_lock);
> -	}
>  }
>  
>  static void apic_send_ipi(struct kvm_lapic *apic)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 48567fa..de99b84 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2010,10 +2010,16 @@ static int kvm_vm_ioctl_get_irqchip(struct kvm *kvm, struct kvm_irqchip *chip)
>  			&pic_irqchip(kvm)->pics[1],
>  			sizeof(struct kvm_pic_state));
>  		break;
> -	case KVM_IRQCHIP_IOAPIC:
> -		memcpy(&chip->chip.ioapic,
> -			ioapic_irqchip(kvm),
> -			sizeof(struct kvm_ioapic_state));
> +	case KVM_IRQCHIP_IOAPIC: {
> +		struct kvm_ioapic *ioapic = ioapic_irqchip(kvm);
> +		if (ioapic) {
> +			spin_lock(&ioapic->lock);
> +			memcpy(&chip->chip.ioapic, ioapic,
> +			       sizeof(struct kvm_ioapic_state));
> +			spin_unlock(&ioapic->lock);
> +		} else
> +			r = -EINVAL;
> +	}
>  		break;
>  	default:
>  		r = -EINVAL;
> @@ -2042,12 +2048,16 @@ static int kvm_vm_ioctl_set_irqchip(struct kvm *kvm, struct kvm_irqchip *chip)
>  			sizeof(struct kvm_pic_state));
>  		spin_unlock(&pic_irqchip(kvm)->lock);
>  		break;
> -	case KVM_IRQCHIP_IOAPIC:
> -		mutex_lock(&kvm->irq_lock);
> -		memcpy(ioapic_irqchip(kvm),
> -			&chip->chip.ioapic,
> -			sizeof(struct kvm_ioapic_state));
> -		mutex_unlock(&kvm->irq_lock);
> +	case KVM_IRQCHIP_IOAPIC: {
> +		struct kvm_ioapic *ioapic = ioapic_irqchip(kvm);
> +		if (ioapic) {
> +			spin_lock(&ioapic->lock);
> +			memcpy(ioapic, &chip->chip.ioapic,
> +			       sizeof(struct kvm_ioapic_state));
> +			spin_unlock(&ioapic->lock);
> +		} else
> +			r = -EINVAL;
> +	}
>  		break;
>  	default:
>  		r = -EINVAL;
> diff --git a/virt/kvm/ioapic.c b/virt/kvm/ioapic.c
> index fa05f67..300ee3b 100644
> --- a/virt/kvm/ioapic.c
> +++ b/virt/kvm/ioapic.c
> @@ -182,6 +182,7 @@ int kvm_ioapic_set_irq(struct kvm_ioapic *ioapic, int irq, int level)
>  	union kvm_ioapic_redirect_entry entry;
>  	int ret = 1;
>  
> +	spin_lock(&ioapic->lock);
>  	if (irq >= 0 && irq < IOAPIC_NUM_PINS) {
>  		entry = ioapic->redirtbl[irq];
>  		level ^= entry.fields.polarity;
> @@ -196,34 +197,44 @@ int kvm_ioapic_set_irq(struct kvm_ioapic *ioapic, int irq, int level)
>  		}
>  		trace_kvm_ioapic_set_irq(entry.bits, irq, ret == 0);
>  	}
> +	spin_unlock(&ioapic->lock);
> +
>  	return ret;
>  }
>  
> -static void __kvm_ioapic_update_eoi(struct kvm_ioapic *ioapic, int pin,
> -				    int trigger_mode)
> +static void __kvm_ioapic_update_eoi(struct kvm_ioapic *ioapic, int vector,
> +				     int trigger_mode)
>  {
> -	union kvm_ioapic_redirect_entry *ent;
> +	int i;
> +
> +	for (i = 0; i < IOAPIC_NUM_PINS; i++) {
> +		union kvm_ioapic_redirect_entry *ent = &ioapic->redirtbl[i];
> +
> +		if (ent->fields.vector != vector)
> +			continue;
>  
> -	ent = &ioapic->redirtbl[pin];
> +		/* ack notifier may call back into ioapic via kvm_set_irq()  */
> +		spin_unlock(&ioapic->lock);
> +		kvm_notify_acked_irq(ioapic->kvm, KVM_IRQCHIP_IOAPIC, i);
> +		spin_lock(&ioapic->lock);

While traversing the IOAPIC pins you drop the lock and acquire it again,
which is error prone: what if the redirection table is changed in
between, for example?

Also, irq_lock was held during ack and mask notifier callbacks, so they
(the callbacks) did not have to worry about concurrency.

You questioned the purpose of irq_lock earlier, and thats what it is:
synchronization between pic and ioapic blur at the irq notifiers.

Using RCU to synchronize ack/mask notifier list walk versus list
addition is good, but i'm not entirely sure that smaller locks like you
are proposing is a benefit long term (things become much more tricky).

Maybe it is the only way forward (and so you push this complexity to the
ack/mask notifiers), but i think it can be avoided until its proven that
there is contention on irq_lock (which is reduced by faster search).


> -	kvm_notify_acked_irq(ioapic->kvm, KVM_IRQCHIP_IOAPIC, pin);
> +		if (trigger_mode != IOAPIC_LEVEL_TRIG)
> +			continue;
>  
> -	if (trigger_mode == IOAPIC_LEVEL_TRIG) {
>  		ASSERT(ent->fields.trig_mode == IOAPIC_LEVEL_TRIG);
>  		ent->fields.remote_irr = 0;
> -		if (!ent->fields.mask && (ioapic->irr & (1 << pin)))
> -			ioapic_service(ioapic, pin);
> +		if (!ent->fields.mask && (ioapic->irr & (1 << i)))
> +			ioapic_service(ioapic, i);
>  	}
>  }
>  
>  void kvm_ioapic_update_eoi(struct kvm *kvm, int vector, int trigger_mode)
>  {
>  	struct kvm_ioapic *ioapic = kvm->arch.vioapic;
> -	int i;
>  
> -	for (i = 0; i < IOAPIC_NUM_PINS; i++)
> -		if (ioapic->redirtbl[i].fields.vector == vector)
> -			__kvm_ioapic_update_eoi(ioapic, i, trigger_mode);
> +	spin_lock(&ioapic->lock);
> +	__kvm_ioapic_update_eoi(ioapic, vector, trigger_mode);
> +	spin_unlock(&ioapic->lock);
>  }
>  
>  static inline struct kvm_ioapic *to_ioapic(struct kvm_io_device *dev)
> @@ -248,8 +259,8 @@ static int ioapic_mmio_read(struct kvm_io_device *this, gpa_t addr, int len,
>  	ioapic_debug("addr %lx\n", (unsigned long)addr);
>  	ASSERT(!(addr & 0xf));	/* check alignment */
>  
> -	mutex_lock(&ioapic->kvm->irq_lock);
>  	addr &= 0xff;
> +	spin_lock(&ioapic->lock);
>  	switch (addr) {
>  	case IOAPIC_REG_SELECT:
>  		result = ioapic->ioregsel;
> @@ -263,6 +274,8 @@ static int ioapic_mmio_read(struct kvm_io_device *this, gpa_t addr, int len,
>  		result = 0;
>  		break;
>  	}
> +	spin_unlock(&ioapic->lock);
> +
>  	switch (len) {
>  	case 8:
>  		*(u64 *) val = result;
> @@ -275,7 +288,6 @@ static int ioapic_mmio_read(struct kvm_io_device *this, gpa_t addr, int len,
>  	default:
>  		printk(KERN_WARNING "ioapic: wrong length %d\n", len);
>  	}
> -	mutex_unlock(&ioapic->kvm->irq_lock);
>  	return 0;
>  }
>  
> @@ -291,15 +303,15 @@ static int ioapic_mmio_write(struct kvm_io_device *this, gpa_t addr, int len,
>  		     (void*)addr, len, val);
>  	ASSERT(!(addr & 0xf));	/* check alignment */
>  
> -	mutex_lock(&ioapic->kvm->irq_lock);
>  	if (len == 4 || len == 8)
>  		data = *(u32 *) val;
>  	else {
>  		printk(KERN_WARNING "ioapic: Unsupported size %d\n", len);
> -		goto unlock;
> +		return 0;
>  	}
>  
>  	addr &= 0xff;
> +	spin_lock(&ioapic->lock);
>  	switch (addr) {
>  	case IOAPIC_REG_SELECT:
>  		ioapic->ioregsel = data;
> @@ -310,15 +322,14 @@ static int ioapic_mmio_write(struct kvm_io_device *this, gpa_t addr, int len,
>  		break;
>  #ifdef	CONFIG_IA64
>  	case IOAPIC_REG_EOI:
> -		kvm_ioapic_update_eoi(ioapic->kvm, data, IOAPIC_LEVEL_TRIG);
> +		__kvm_ioapic_update_eoi(ioapic, data, IOAPIC_LEVEL_TRIG);
>  		break;
>  #endif
>  
>  	default:
>  		break;
>  	}
> -unlock:
> -	mutex_unlock(&ioapic->kvm->irq_lock);
> +	spin_unlock(&ioapic->lock);
>  	return 0;
>  }
>  
> @@ -347,6 +358,7 @@ int kvm_ioapic_init(struct kvm *kvm)
>  	ioapic = kzalloc(sizeof(struct kvm_ioapic), GFP_KERNEL);
>  	if (!ioapic)
>  		return -ENOMEM;
> +	spin_lock_init(&ioapic->lock);
>  	kvm->arch.vioapic = ioapic;
>  	kvm_ioapic_reset(ioapic);
>  	kvm_iodevice_init(&ioapic->dev, &ioapic_mmio_ops);
> diff --git a/virt/kvm/ioapic.h b/virt/kvm/ioapic.h
> index 7080b71..557107e 100644
> --- a/virt/kvm/ioapic.h
> +++ b/virt/kvm/ioapic.h
> @@ -44,6 +44,7 @@ struct kvm_ioapic {
>  	struct kvm_io_device dev;
>  	struct kvm *kvm;
>  	void (*ack_notifier)(void *opaque, int irq);
> +	spinlock_t lock;
>  };
>  
>  #ifdef DEBUG
> -- 
> 1.6.2.1
> 
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