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

[Michael S. Tsirkin]

On Tue, Jul 17, 2012 at 08:21:51AM -0600, Alex Williamson wrote:
> 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.

Well you are asked to clear some id and level was 1. So we know
interrupt was asserted. Either we clear it or we don't. No?

> > 
> > 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,

On x86 they are:
static __always_inline void
set_bit(unsigned int nr, volatile unsigned long *addr)
{
        if (IS_IMMEDIATE(nr)) {
                asm volatile(LOCK_PREFIX "orb %1,%0"
                        : CONST_MASK_ADDR(nr, addr)
                        : "iq" ((u8)CONST_MASK(nr))
                        : "memory");
        } else {
                asm volatile(LOCK_PREFIX "bts %1,%0"
                        : BITOP_ADDR(addr) : "Ir" (nr) : "memory");
        }
}

> 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).

For me it's not a performance thing. IMO code is cleaner without this locking:
we add a lock but only use it in some cases, so the rules become really
complex.  And current code looks buggy if yes we need to fix it somehow.

-- 
MST
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