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    SubjectRe: [kvm-devel] [PATCH 3/3] virtio PCI device
    From
    Anthony Liguori wrote:
    > This is a PCI device that implements a transport for virtio. It allows virtio
    > devices to be used by QEMU based VMMs like KVM or Xen.
    >
    > ....
    >
    While it's a little premature, we can start thinking of irq path
    improvements.
    The current patch acks a private isr and afterwards apic eoi will also
    be hit since its
    a level trig irq. This means 2 vmexits per irq.
    We can start with regular pci irqs and move afterwards to msi.
    Some other ugly hack options [we're better use msi]:
    - Read the eoi directly from apic and save the first private isr ack
    - Convert the specific irq line to edge triggered and dont share it
    What do you guys think?
    > +/* A small wrapper to also acknowledge the interrupt when it's handled.
    > + * I really need an EIO hook for the vring so I can ack the interrupt once we
    > + * know that we'll be handling the IRQ but before we invoke the callback since
    > + * the callback may notify the host which results in the host attempting to
    > + * raise an interrupt that we would then mask once we acknowledged the
    > + * interrupt. */
    > +static irqreturn_t vp_interrupt(int irq, void *opaque)
    > +{
    > + struct virtio_pci_device *vp_dev = opaque;
    > + struct virtio_pci_vq_info *info;
    > + irqreturn_t ret = IRQ_NONE;
    > + u8 isr;
    > +
    > + /* reading the ISR has the effect of also clearing it so it's very
    > + * important to save off the value. */
    > + isr = ioread8(vp_dev->ioaddr + VIRTIO_PCI_ISR);
    > +
    > + /* It's definitely not us if the ISR was not high */
    > + if (!isr)
    > + return IRQ_NONE;
    > +
    > + spin_lock(&vp_dev->lock);
    > + list_for_each_entry(info, &vp_dev->virtqueues, node) {
    > + if (vring_interrupt(irq, info->vq) == IRQ_HANDLED)
    > + ret = IRQ_HANDLED;
    > + }
    > + spin_unlock(&vp_dev->lock);
    > +
    > + return ret;
    > +}
    >

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