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

From: Paolo Bonzini
Date: Wed Oct 28 2015 - 13:05:35 EST

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

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at