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Date: Tue, 17 Jul 2012 08:21:51 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: avi@...hat.com, gleb@...hat.com, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org, jan.kiszka@...mens.com
Subject: Re: [PATCH v5 3/4] kvm: Create kvm_clear_irq()
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@...hat.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?
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).
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