Re: [kvm-devel] [PATCH 3/3] virtio PCI device

[Anthony Liguori <aliguori@us.ibm.com>]

Avi Kivity wrote:
> 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.
> >
> > +
> > +/* the notify function used when creating a virt queue */
> > +static void vp_notify(struct virtqueue *vq)
> > +{
> > +    struct virtio_pci_device *vp_dev = to_vp_device(vq->vdev);
> > +    struct virtio_pci_vq_info *info = vq->priv;
> > +
> > +    /* we write the queue's selector into the notification register to
> > +     * signal the other end */
> > +    iowrite16(info->queue_index, vp_dev->ioaddr + 
> > VIRTIO_PCI_QUEUE_NOTIFY);
> > +}
> >   
>
> This means we can't kick multiple queues with one exit.

There is no interface in virtio currently to batch multiple queue 
notifications so the only way one could do this AFAICT is to use a timer 
to delay the notifications.  Were you thinking of something else?

> I'd also like to see a hypercall-capable version of this (but that can 
> wait).

That can be a different device.

> > +
> > +/* 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);
> >   
>
> Can this be implemented via shared memory? We're exiting now on every 
> interrupt.

I don't think so.  A vmexit is required to lower the IRQ line.  It may 
be possible to do something clever like set a shared memory value that's 
checked on every vmexit.  I think it's very unlikely that it's worth it 
though.

> > +    return ret;
> > +}
> > +
> > +/* the config->find_vq() implementation */
> > +static struct virtqueue *vp_find_vq(struct virtio_device *vdev, 
> > unsigned index,
> > +                    bool (*callback)(struct virtqueue *vq))
> > +{
> > +    struct virtio_pci_device *vp_dev = to_vp_device(vdev);
> > +    struct virtio_pci_vq_info *info;
> > +    struct virtqueue *vq;
> > +    int err;
> > +    u16 num;
> > +
> > +    /* Select the queue we're interested in */
> > +    iowrite16(index, vp_dev->ioaddr + VIRTIO_PCI_QUEUE_SEL);
> >   
>
> I would really like to see this implemented as pci config space, with 
> no tricks like multiplexing several virtqueues on one register. 
> Something like the PCI BARs where you have all the register numbers 
> allocated statically to queues.

My first implementation did that.  I switched to using a selector 
because it reduces the amount of PCI config space used and does not 
limit the number of queues defined by the ABI as much.

> > +
> > +    /* Check if queue is either not available or already active. */
> > +    num = ioread16(vp_dev->ioaddr + VIRTIO_PCI_QUEUE_NUM);
> > +    if (!num || ioread32(vp_dev->ioaddr + VIRTIO_PCI_QUEUE_PFN))
> > +        return ERR_PTR(-ENOENT);
> > +
> > +    /* allocate and fill out our structure the represents an active
> > +     * queue */
> > +    info = kmalloc(sizeof(struct virtio_pci_vq_info), GFP_KERNEL);
> > +    if (!info)
> > +        return ERR_PTR(-ENOMEM);
> > +
> > +    info->queue_index = index;
> > +    info->num = num;
> > +
> > +    /* determine the memory needed for the queue and provide the memory
> > +     * location to the host */
> > +    info->n_pages = DIV_ROUND_UP(vring_size(num), PAGE_SIZE);
> > +    info->pages = alloc_pages(GFP_KERNEL | __GFP_ZERO,
> > +                  get_order(info->n_pages));
> > +    if (info->pages == NULL) {
> > +        err = -ENOMEM;
> > +        goto out_info;
> > +    }
> > +
> > +    /* FIXME: is this sufficient for info->n_pages > 1? */
> > +    info->queue = kmap(info->pages);
> > +    if (info->queue == NULL) {
> > +        err = -ENOMEM;
> > +        goto out_alloc_pages;
> > +    }
> > +
> > +    /* activate the queue */
> > +    iowrite32(page_to_pfn(info->pages),
> > +          vp_dev->ioaddr + VIRTIO_PCI_QUEUE_PFN);
> > +          +    /* create the vring */
> > +    vq = vring_new_virtqueue(info->num, vdev, info->queue,
> > +                 vp_notify, callback);
> > +    if (!vq) {
> > +        err = -ENOMEM;
> > +        goto out_activate_queue;
> > +    }
> > +
> > +    vq->priv = info;
> > +    info->vq = vq;
> > +
> > +    spin_lock(&vp_dev->lock);
> > +    list_add(&info->node, &vp_dev->virtqueues);
> > +    spin_unlock(&vp_dev->lock);
> > +
> >   
>
> Is this run only on init? If so the lock isn't needed.

Yes, it's also not stricly needed on cleanup I think.  I left it there 
though for clarity.  I can remove.

Regards,

Anthony Liguori

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