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Message-ID: <56301A87.9030907@redhat.com> Date: Wed, 28 Oct 2015 01:44:55 +0100 From: Paolo Bonzini <pbonzini@...hat.com> To: Yunhong Jiang <yunhong.jiang@...ux.intel.com> Cc: Alex Williamson <alex.williamson@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [RFC PATCH] VFIO: Add a parameter to force nonthread IRQ On 27/10/2015 22:26, Yunhong Jiang wrote: >> > On RT kernels however can you call eventfd_signal from interrupt >> > context? You cannot call spin_lock_irqsave (which can sleep) from a >> > non-threaded interrupt handler, can you? You would need a raw spin lock. > Thanks for pointing this out. Yes, we can't call spin_lock_irqsave on RT > kernel. Will do this way on next patch. But not sure if it's overkill to use > raw_spinlock there since the eventfd_signal is used by other caller also. No, I don't think you can use raw_spinlock there. The problem is not just eventfd_signal, it is especially wake_up_locked_poll. You cannot convert the whole workqueue infrastructure to use raw_spinlock. 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. The KVM implementation would be like this (compare with virt/kvm/eventfd.c): /* Extracted out of irqfd_wakeup */ static int irqfd_wakeup_pollin(struct kvm_kernel_irqfd *irqfd) { ... } /* Extracted out of irqfd_wakeup */ static int irqfd_wakeup_pollhup(struct kvm_kernel_irqfd *irqfd) { ... } static int irqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key) { struct _irqfd *irqfd = container_of(wait, struct _irqfd, wait); unsigned long flags = (unsigned long)key; if (flags & POLLIN) irqfd_wakeup_pollin(irqfd); if (flags & POLLHUP) irqfd_wakeup_pollhup(irqfd); return 0; } static int kvm_arch_irq_bypass_inject( struct irq_bypass_consumer *cons) { struct kvm_kernel_irqfd *irqfd = container_of(cons, struct kvm_kernel_irqfd, consumer); irqfd_wakeup_pollin(irqfd); } Or do you think it would be a hack? The latency improvement might actually be even better than what Yunhong is already reporting. Paolo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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