On Thu, Jul 28, 2022, 11:34 Jinhao Fan <fanjinhao...@ict.ac.cn> wrote:
> at 11:18 PM, Stefan Hajnoczi <stefa...@gmail.com> wrote: > > > I think that is incorrect. QEMU has guest notifier emulation for the > > non-KVM (and non-MSI-X PCI) cases. When there is no irqfd support > > available, QEMU sets up a regular eventfd and calls > > virtio_queue_guest_notifier_read() when it becomes readable. > > Thanks Stefan. I finally understand why there is a `with_irqfd` parameter > for virtio_queue_set_guest_notifier_fd_handler. > > But if `with_irqfd` is false, it seems OK to directly call virtio_irq(). > Why > still bother using an eventfd? Is it for interrupt batching? > virtio_irq() is not thread safe so it cannot be called directly from the IOThread. Bouncing through the eventfd ensures that the virtio_irq() call happens in the QEMU main loop thread with the BQL held. It may be cheaper to use a BH instead of an eventfd when irqfd is not available, but this is a slow path anyway. We might as well reuse the eventfd code that's already there. Stefan >
