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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jul 17, 2012 at 07:56:09AM -0600, Alex Williamson wrote:
> 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.

If not then I don't understand. You clear a bit
in a word. You never change it to 1, do you?

But this brings another question:

static inline int kvm_irq_line_state(unsigned long *irq_state,
                                     int irq_source_id, int level)
{
        /* Logical OR for level trig interrupt */
        if (level)
                set_bit(irq_source_id, irq_state);
        else
                clear_bit(irq_source_id, irq_state);


^^^^^^^^^^^
above uses locked instructions

        return !!(*irq_state);


above doesn't

}


why the insonsistency?

> > > +#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;
> 
>