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    SubjectRe: [PATCH v5 3/4] kvm: Create kvm_clear_irq()
    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;
    >
    >


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