Re: [RFC PATCH 01/12] virtio/s390: use vring_create_virtqueue

Cornelia Huck
2019-04-08T11:01:28Z

On Fri, 5 Apr 2019 01:16:11 +0200
Halil Pasic <pasic@linux.ibm.com> wrote:

> The commit 2a2d1382fe9d ("virtio: Add improved queue allocation API")
> establishes a new way of allocating virtqueues (as a part of the effort
> that taught DMA to virtio rings).
>
> In the future we will want virtio-ccw to use the DMA API as well.
>
> Let us switch from the legacy method of allocating virtqueues to
> vring_create_virtqueue() as the first step into that direction.
>
> Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
> ---
> drivers/s390/virtio/virtio_ccw.c | 27 ++++++++-------------------
> 1 file changed, 8 insertions(+), 19 deletions(-)
>
> diff --git a/drivers/s390/virtio/virtio_ccw.c b/drivers/s390/virtio/virtio_ccw.c
> index 74c328321889..edf4afe2d688 100644
> --- a/drivers/s390/virtio/virtio_ccw.c
> +++ b/drivers/s390/virtio/virtio_ccw.c

> @@ -516,17 +512,10 @@ static struct virtqueue *virtio_ccw_setup_vq(struct virtio_device *vdev,
> err = info->num;
> goto out_err;
> }
> - size = PAGE_ALIGN(vring_size(info->num, KVM_VIRTIO_CCW_RING_ALIGN));
> - info->queue = alloc_pages_exact(size, GFP_KERNEL | __GFP_ZERO);
> - if (info->queue == NULL) {
> - dev_warn(&vcdev->cdev->dev, "no queue\n");
> - err = -ENOMEM;
> - goto out_err;
> - }
> + vq = vring_create_virtqueue(i, info->num, KVM_VIRTIO_CCW_RING_ALIGN,
> + vdev, true, true, ctx,

This second true means 'may_reduce_num'. Looking at the vring code, it
seems that this parameter is never checked; the code will try to
allocate a smaller queue if it can't get the requested size in any
case... this will probably be a problem for legacy virtio-pci, which
explicitly sets may_reduce_num to false. (I can try to come up with a
patch to fix that.)

> + virtio_ccw_kvm_notify, callback, name);
>
> - vq = vring_new_virtqueue(i, info->num, KVM_VIRTIO_CCW_RING_ALIGN, vdev,
> - true, ctx, info->queue, virtio_ccw_kvm_notify,
> - callback, name);
> if (!vq) {
> /* For now, we fail if we can't get the requested size. */
> dev_warn(&vcdev->cdev->dev, "no vq\n");
> @@ -534,15 +523,17 @@ static struct virtqueue *virtio_ccw_setup_vq(struct virtio_device *vdev,
> goto out_err;
> }
>
> +

Extra blank line :)

> /* Register it with the host. */
> + queue = virtqueue_get_desc_addr(vq);
> if (vcdev->revision == 0) {
> - info->info_block->l.queue = (__u64)info->queue;
> + info->info_block->l.queue = queue;
> info->info_block->l.align = KVM_VIRTIO_CCW_RING_ALIGN;
> info->info_block->l.index = i;
> info->info_block->l.num = info->num;

You always fill in the size requested by the host, but the actual size
may be smaller (see above). I don't think that is allowed for revision
0 (which implies !virtio-1). You probably need to call
vring_create_virtqueue with may_reduce_num=false for revision 0 (and
wait for the generic vring code to be fixed...)

> ccw->count = sizeof(info->info_block->l);
> } else {
> - info->info_block->s.desc = (__u64)info->queue;
> + info->info_block->s.desc = queue;
> info->info_block->s.index = i;
> info->info_block->s.num = info->num;

Here, you need to obtain the actual number via
virtqueue_get_vring_size().

> info->info_block->s.avail = (__u64)virtqueue_get_avail(vq);