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

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

On Wed, 2015-10-28 at 18:05 +0100, Paolo Bonzini wrote:
> 
> 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.

I'm confused by this.  The problem with adding IRQF_NO_THREAD to our
current handler is that it hits the spinlock that can sleep in
eventfd_signal() and the waitqueue further down the stack before we get
to the irqfd.  So if we split to a non-threaded handler vs a threaded
handler, where the non-threaded handler either returns IRQ_HANDLED or
IRQ_WAKE_THREAD to queue the threaded handler, there's only so much that
the non-threaded handler can do before we start running into the same
problem.  I think that means that the non-threaded handler needs to
return IRQ_WAKE_THREAD if we need to use the current eventfd_signal()
path, such as if the bypass path is not available.  If we can get
through the bypass path and the KVM irqfd side is safe for the
non-threaded handler, inject succeeds and we return IRQ_HANDLED, right?
Thanks,

Alex