Re: [PATCH v5 3/4] kvm: Create kvm_clear_irq()

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 2012-07-17 at 13:14 +0300, Michael S. Tsirkin wrote:
> On Mon, Jul 16, 2012 at 02:34:03PM -0600, Alex Williamson wrote:
> > This is an alternative to kvm_set_irq(,,,0) which returns the previous
> > assertion state of the interrupt and does nothing if it isn't changed.
> > 
> > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > ---
> > 
> >  include/linux/kvm_host.h |    3 ++
> >  virt/kvm/irq_comm.c      |   78 ++++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 81 insertions(+)
> > 
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index a7661c0..6c168f1 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -219,6 +219,8 @@ struct kvm_kernel_irq_routing_entry {
> >  	u32 type;
> >  	int (*set)(struct kvm_kernel_irq_routing_entry *e,
> >  		   struct kvm *kvm, int irq_source_id, int level);
> > +	int (*clear)(struct kvm_kernel_irq_routing_entry *e,
> > +		     struct kvm *kvm, int irq_source_id);
> >  	union {
> >  		struct {
> >  			unsigned irqchip;
> > @@ -629,6 +631,7 @@ void kvm_get_intr_delivery_bitmask(struct kvm_ioapic *ioapic,
> >  				   unsigned long *deliver_bitmask);
> >  #endif
> >  int kvm_set_irq(struct kvm *kvm, int irq_source_id, u32 irq, int level);
> > +int kvm_clear_irq(struct kvm *kvm, int irq_source_id, u32 irq);
> >  int kvm_set_msi(struct kvm_kernel_irq_routing_entry *irq_entry, struct kvm *kvm,
> >  		int irq_source_id, int level);
> >  void kvm_notify_acked_irq(struct kvm *kvm, unsigned irqchip, unsigned pin);
> > diff --git a/virt/kvm/irq_comm.c b/virt/kvm/irq_comm.c
> > index 5afb431..76e8f22 100644
> > --- a/virt/kvm/irq_comm.c
> > +++ b/virt/kvm/irq_comm.c
> > @@ -68,6 +68,42 @@ static int kvm_set_ioapic_irq(struct kvm_kernel_irq_routing_entry *e,
> >  	return kvm_ioapic_set_irq(ioapic, e->irqchip.pin, level);
> >  }
> >  
> > +static inline int kvm_clear_irq_line_state(unsigned long *irq_state,
> > +					    int irq_source_id)
> > +{
> > +	return !!test_and_clear_bit(irq_source_id, irq_state);
> > +}
> > +
> > +static int kvm_clear_pic_irq(struct kvm_kernel_irq_routing_entry *e,
> > +			     struct kvm *kvm, int irq_source_id)
> > +{
> > +#ifdef CONFIG_X86
> > +	struct kvm_pic *pic = pic_irqchip(kvm);
> > +	int level = kvm_clear_irq_line_state(&pic->irq_states[e->irqchip.pin],
> > +					     irq_source_id);
> > +	if (level)
> > +		kvm_pic_set_irq(pic, e->irqchip.pin,
> > +				!!pic->irq_states[e->irqchip.pin]);
> > +	return level;
> 
> I think I begin to understand: if (level) checks it was previously set,
> and then we clear if needed?

It's actually very simple, if we change anything in irq_states, then
update via the chip specific set_irq function.

>  I think it's worthwhile to rename
> level to orig_level and rewrite as:
> 
> 	if (orig_level && !pic->irq_states[e->irqchip.pin])
> 		kvm_pic_set_irq(pic, e->irqchip.pin, 0);
> 
> This both makes the logic clear without need for comments and
> saves some cycles on pic in case nothing actually changed.

That may work, but it's not actually the same thing.  kvm_set_irq(,,,0)
will clear the bit and call kvm_pic_set_irq with the new irq_states
value, whether it's 0 or 1.  The optimization I make is to only call
kvm_pic_set_irq if we've "changed" irq_states.  You're taking that one
step further to "changed and is now 0".  I don't know if that's correct
behavior.

> > +#else
> > +	return -1;
> > +#endif
> > +}
> > +
> > +static int kvm_clear_ioapic_irq(struct kvm_kernel_irq_routing_entry *e,
> > +				struct kvm *kvm, int irq_source_id)
> > +{
> > +	struct kvm_ioapic *ioapic = kvm->arch.vioapic;
> > +	int level;
> > +
> > +	level = kvm_clear_irq_line_state(&ioapic->irq_states[e->irqchip.pin],
> > +					 irq_source_id);
> > +	if (level)
> > +		kvm_ioapic_set_irq(ioapic, e->irqchip.pin,
> > +				   !!ioapic->irq_states[e->irqchip.pin]);
> > +	return level;
> > +}
> > +
> >  inline static bool kvm_is_dm_lowest_prio(struct kvm_lapic_irq *irq)
> >  {
> >  #ifdef CONFIG_IA64
> > @@ -190,6 +226,45 @@ int kvm_set_irq(struct kvm *kvm, int irq_source_id, u32 irq, int level)
> >  	return ret;
> >  }
> >  
> > +/*
> > + * Return value:
> > + *  < 0   Error
> > + *  = 0   Interrupt was not set, did nothing
> > + *  > 0   Interrupt was pending, cleared
> > + */
> > +int kvm_clear_irq(struct kvm *kvm, int irq_source_id, u32 irq)
> > +{
> > +	struct kvm_kernel_irq_routing_entry *e, irq_set[KVM_NR_IRQCHIPS];
> > +	int ret = -EINVAL, i = 0;
> > +	struct kvm_irq_routing_table *irq_rt;
> > +	struct hlist_node *n;
> > +
> > +	/* Not possible to detect if the guest uses the PIC or the
> > +	 * IOAPIC.  So clear the bit in both. The guest will ignore
> > +	 * writes to the unused one.
> > +	 */
> > +	rcu_read_lock();
> > +	irq_rt = rcu_dereference(kvm->irq_routing);
> > +	if (irq < irq_rt->nr_rt_entries)
> > +		hlist_for_each_entry(e, n, &irq_rt->map[irq], link)
> > +			irq_set[i++] = *e;
> > +	rcu_read_unlock();
> > +
> > +	while (i--) {
> > +		int r = -EINVAL;
> > +
> > +		if (irq_set[i].clear)
> > +			r = irq_set[i].clear(&irq_set[i], kvm, irq_source_id);
> > +
> > +		if (r < 0)
> > +			continue;
> > +
> > +		ret = r + ((ret < 0) ? 0 : ret);
> > +	}
> > +
> > +	return ret;
> > +}
> > +
> >  void kvm_notify_acked_irq(struct kvm *kvm, unsigned irqchip, unsigned pin)
> >  {
> >  	struct kvm_irq_ack_notifier *kian;
> > @@ -344,16 +419,19 @@ static int setup_routing_entry(struct kvm_irq_routing_table *rt,
> >  		switch (ue->u.irqchip.irqchip) {
> >  		case KVM_IRQCHIP_PIC_MASTER:
> >  			e->set = kvm_set_pic_irq;
> > +			e->clear = kvm_clear_pic_irq;
> >  			max_pin = 16;
> >  			break;
> >  		case KVM_IRQCHIP_PIC_SLAVE:
> >  			e->set = kvm_set_pic_irq;
> > +			e->clear = kvm_clear_pic_irq;
> >  			max_pin = 16;
> >  			delta = 8;
> >  			break;
> >  		case KVM_IRQCHIP_IOAPIC:
> >  			max_pin = KVM_IOAPIC_NUM_PINS;
> >  			e->set = kvm_set_ioapic_irq;
> > +			e->clear = kvm_clear_ioapic_irq;
> >  			break;
> >  		default:
> >  			goto out;