Re: [Qemu-devel] [PATCH 3/6] virtio for QEMU

[Dor Laor <dor.laor@gmail.com>]

On Sat, 2008-03-29 at 16:55 -0500, Anthony Liguori wrote:
> This patch introduces virtio support over PCI.  virtio is a generic virtual IO
> framework for Linux first introduced in 2.6.23.  Since 2.6.25, virtio has
> supported a PCI transport which this patch implements.
> 
> Since the last time these patches were posted to qemu-devel, I've reworked it
> to use the proper access functions to manipulate guest memory.
> 
> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

It's will be great to drop the nasty hacks :)
Do you still get 1G net performance using the extra copy from tap
(memcpy_to_iovector)?

[snip]

> +static uint32_t vring_desc_len(VirtQueue *vq, unsigned int i)
> +{

Below there were place you did use offsetof(vq->vring.desc[i], len) so
we better be consistent + its nicer

> +    return ldl_phys(vq->vring.desc + i * sizeof(VRingDesc) + 
> +		    offsetof(VRingDesc, len));
> +}
> +

[snip]

> +VirtQueueElement *virtqueue_pop(VirtQueue *vq)
> +{
> +    unsigned int i, head;
> +    unsigned int position;
> +    VirtQueueElement *elem;
> +
> +    /* Check it isn't doing very strange things with descriptor numbers. */
> +    if ((uint16_t)(vring_avail_idx(vq) - vq->last_avail_idx) > vq->vring.num)
> +	errx(1, "Guest moved used index from %u to %u",
> +	     vq->last_avail_idx, vring_avail_idx(vq));
> +
> +    /* If there's nothing new since last we looked, return invalid. */
> +    if (vring_avail_idx(vq) == vq->last_avail_idx)
> +	return NULL;
> +
> +    /* Grab the next descriptor number they're advertising, and increment
> +     * the index we've seen. */
> +    head = vring_avail_ring(vq, vq->last_avail_idx++ % vq->vring.num);
> +
> +    /* If their number is silly, that's a fatal mistake. */
> +    if (head >= vq->vring.num)
> +	errx(1, "Guest says index %u is available", head);
> +
> +    /* When we start there are none of either input nor output. */
> +    position = 0;
> +
> +    elem = qemu_mallocz(sizeof(VirtQueueElement));
> +
> +    elem->phys_in = qemu_mallocz(sizeof(PhysIOVector) +
> +				 vq->vring.num * sizeof(PhysIOVectorElement));
> +    elem->phys_out = qemu_mallocz(sizeof(PhysIOVector) +
> +				  vq->vring.num * sizeof(PhysIOVectorElement));

I was wondering whether it can be optimized since vring.num is sometimes
512 so and we can either use a pool of these or calculate the vring.num
from the descriptors but it seems like your way is the best.

> +
> +    i = head;
> +    do {
> +	PhysIOVectorElement *sge;
> +
> +	if (vring_desc_flags(vq, i) & VRING_DESC_F_WRITE)
> +	    sge = &elem->phys_in->sg[elem->phys_in->num++];
> +	else
> +	    sge = &elem->phys_out->sg[elem->phys_out->num++];
> +
> +	/* Grab the first descriptor, and check it's OK. */
> +	sge->len = vring_desc_len(vq, i);
> +	sge->base = vring_desc_addr(vq, i);
> +
> +	/* If we've got too many, that implies a descriptor loop. */
> +	if ((elem->phys_in->num + elem->phys_out->num) > vq->vring.num)
> +	    errx(1, "Looped descriptor");
> +    } while ((i = virtqueue_next_desc(vq, i)) != vq->vring.num);
> +
> +    elem->virt_in = pci_device_dma_map(&vq->vdev->pci_dev, elem->phys_in);
> +    elem->virt_out = pci_device_dma_map(&vq->vdev->pci_dev, elem->phys_out);
> +    elem->index = head;
> +
> +    if (elem->virt_in == NULL || elem->virt_out == NULL)
> +	errx(1, "Bad DMA");
> +
> +    return elem;
> +}
> +
> +

The name below is a bit misleading since when enable is true you
actually set no_notify.
So I name it something like virtio_vring_set_no_notify(...) or similar.


> +void virtio_ring_set_used_notify(VirtQueue *vq, int enable)
> +{
> +    if (enable)
> +	vring_used_set_flag(vq, VRING_USED_F_NO_NOTIFY);
> +    else
> +	vring_used_unset_flag(vq, VRING_USED_F_NO_NOTIFY);
> +}
> +

Cheers,
Dor