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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
