Re: [PATCH v5 1/4] kvm: Extend irqfd to support level interrupts

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 2012-07-18 at 15:07 +0300, Michael S. Tsirkin wrote:
> On Wed, Jul 18, 2012 at 02:48:44PM +0300, Gleb Natapov wrote:
> > On Wed, Jul 18, 2012 at 02:39:10PM +0300, Michael S. Tsirkin wrote:
> > > On Wed, Jul 18, 2012 at 02:22:19PM +0300, Michael S. Tsirkin wrote:
> > > > > > > > > > So as was discussed kvm_set_irq under spinlock is bad for scalability
> > > > > > > > > > with multiple VCPUs.  Why do we need a spinlock simply to protect
> > > > > > > > > > level_asserted?  Let's use an atomic test and set/test and clear and the
> > > > > > > > > > problem goes away.
> > > > > > > > > > 
> > > > > > > > > That sad reality is that for level interrupt we already scan all vcpus
> > > > > > > > > under spinlock.
> > > > > > > > 
> > > > > > > > Where?
> > > > > > > > 
> > > > > > > ioapic
> > > > > > 
> > > > > > $ grep kvm_for_each_vcpu virt/kvm/ioapic.c
> > > > > > $
> > > > > > 
> > > > > > ?
> > > > > > 
> > > > > 
> > > > > Come on Michael. You can do better than grep and actually look at what
> > > > > code does. The code that loops over all vcpus while delivering an irq is
> > > > > in kvm_irq_delivery_to_apic(). Now grep for that.
> > > > 
> > > > Hmm, I see, it's actually done for edge if injected from ioapic too,
> > > > right?
> > > > 
> > > > So set_irq does a linear scan, and for each matching CPU it calls
> > > > kvm_irq_delivery_to_apic which is another scan?
> > > > So it's actually N^2 worst case for a broadcast?
> > > 
> > > No it isn't, I misread the code.
> > > 
> > > 
> > > Anyway, maybe not trivially but this looks fixable to me: we could drop
> > > the ioapic lock before calling kvm_irq_delivery_to_apic.
> > > 
> > May be, may be not. Just saying "lets drop lock whenever we don't feel
> > like holding one" does not cut it.
> 
> One thing we do is set remote_irr if interrupt was injected.
> I agree these things are tricky.
> 
> One other question:
> 
> static int ioapic_service(struct kvm_ioapic *ioapic, unsigned int idx)
> {
>         union kvm_ioapic_redirect_entry *pent;
>         int injected = -1;
> 
>         pent = &ioapic->redirtbl[idx];
> 
>         if (!pent->fields.mask) {
>                 injected = ioapic_deliver(ioapic, idx);
>                 if (injected && pent->fields.trig_mode == IOAPIC_LEVEL_TRIG)
>                         pent->fields.remote_irr = 1;
>         }
> 
>         return injected;
> }
> 
> 
> This if (injected) looks a bit strange since ioapic_deliver returns
> -1 if no matching destinations. Should be if (injected > 0)?
> 
> 
> 
> > Back to original point though current
> > situation is that calling kvm_set_irq() under spinlock is not worse for
> > scalability than calling it not under one.
> 
> Yes. Still the specific use can just use an atomic flag,
> lock+bool is not needed, and we won't need to undo it later.


Actually, no, replacing it with an atomic is racy.

CPU0 (inject)                       CPU1 (EOI)
atomic_cmpxchg(&asserted, 0, 1)
                                    atomic_cmpxchg(&asserted, 1, 0)
                                    kvm_set_irq(0)
kvm_set_irq(1)
                                    eventfd_signal

The interrupt is now stuck on until another interrupt is injected.