Re: [RFC PATCH] VFIO: Add a parameter to force nonthread IRQ

[Paolo Bonzini <pbonzini@redhat.com>]

On 28/10/2015 17:00, Alex Williamson wrote:
> > Alex, would it make sense to use the IRQ bypass infrastructure always,
> > not just for VT-d, to do the MSI injection directly from the VFIO
> > interrupt handler and bypass the eventfd?  Basically this would add an
> > RCU-protected list of consumers matching the token to struct
> > irq_bypass_producer, and a
> > 
> > 	int (*inject)(struct irq_bypass_consumer *);
> > 
> > callback to struct irq_bypass_consumer.  If any callback returns true,
> > the eventfd is not signaled.
>
> Yeah, that might be a good idea, it's probably more plausible than
> making the eventfd_signal() code friendly to call from hard interrupt
> context.  On the vfio side can we use request_threaded_irq() directly
> for this?

I don't know if that gives you a non-threaded IRQ with the real-time
kernel...  CCing Marcelo to get some insight.

> Making the hard irq handler return IRQ_HANDLED if we can use
> the irq bypass manager or IRQ_WAKE_THREAD if we need to use the eventfd.
> I think we need some way to get back to irq thread context to use
> eventfd_signal().

The irqfd is already able to schedule a work item, because it runs with
interrupts disabled, so I think we can always return IRQ_HANDLED.

There's another little complication.  Right now, only x86 has
kvm_set_msi_inatomic.  We should merge kvm_set_msi_inatomic,
kvm_set_irq_inatomic and kvm_arch_set_irq.

Some cleanups are needed there; the flow between the functions is really
badly structured because the API grew somewhat by accretion.  I'll get
to it next week or on the way back to Italy.

> Would we ever not want to use the direct bypass
> manager path if available?  Thanks,

I don't think so.  KVM always registers itself as a consumer, even if
there is no VT-d posted interrupts.  add_producer simply returns -EINVAL
then.

Paolo