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

[Alex Williamson]

On Tue, 2012-07-17 at 17:08 +0300, Michael S. Tsirkin wrote:
> 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@xxxxxxxxxx>
> > > > ---
> > > > 
> > > >  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?

Correct, but kvm_set_irq(,,,0) may call kvm_pic_set_irq(,,1) if other
source IDs are still asserting the interrupt.  Your proposal assumes
that unless irq_states is also 0 we don't need to call kvm_pic_set_irq,
and I don't know if that's correct.

> 
> 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?

Note that set/clear_bit are not locked instructions, but atomic
instructions and it could be argued that reading the value is also
atomic.  At least that was my guess when I stumbled across the same
yesterday.  IMHO, we're going off into the weeds again with these last
two patches.  It may be a valid optimization, but it really has no
bearing on the meat of the series (and afaict, no significant
performance difference either).



--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/