Re: [PATCH v12 08/40] virtio_ring: split: extract the logic of alloc queue

Xuan Zhuo
2022-07-22T03:58:01Z

On Thu, 21 Jul 2022 17:13:49 +0800, Jason Wang <jasowang@redhat.com> wrote:
>
> 在 2022/7/20 11:04, Xuan Zhuo 写道:
> > Separate the logic of split to create vring queue.
> >
> > This feature is required for subsequent virtuqueue reset vring.
> >
> > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > ---
> > drivers/virtio/virtio_ring.c | 68 ++++++++++++++++++++++--------------
> > 1 file changed, 42 insertions(+), 26 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index c94c5461e702..c7971438bb2c 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -950,28 +950,19 @@ static void vring_free_split(struct vring_virtqueue_split *vring_split,
> > kfree(vring_split->desc_extra);
> > }
> >
> > -static struct virtqueue *vring_create_virtqueue_split(
> > - unsigned int index,
> > - unsigned int num,
> > - unsigned int vring_align,
> > - struct virtio_device *vdev,
> > - bool weak_barriers,
> > - bool may_reduce_num,
> > - bool context,
> > - bool (*notify)(struct virtqueue *),
> > - void (*callback)(struct virtqueue *),
> > - const char *name)
> > +static int vring_alloc_queue_split(struct vring_virtqueue_split *vring_split,
> > + struct virtio_device *vdev,
> > + u32 num,
> > + unsigned int vring_align,
> > + bool may_reduce_num)
> > {
> > - struct virtqueue *vq;
> > void *queue = NULL;
> > dma_addr_t dma_addr;
> > - size_t queue_size_in_bytes;
> > - struct vring vring;
> >
> > /* We assume num is a power of 2. */
> > if (num & (num - 1)) {
> > dev_warn(&vdev->dev, "Bad virtqueue length %u\n", num);
> > - return NULL;
> > + return -EINVAL;
> > }
> >
> > /* TODO: allocate each queue chunk individually */
> > @@ -982,11 +973,11 @@ static struct virtqueue *vring_create_virtqueue_split(
> > if (queue)
> > break;
> > if (!may_reduce_num)
> > - return NULL;
> > + return -ENOMEM;
> > }
> >
> > if (!num)
> > - return NULL;
> > + return -ENOMEM;
> >
> > if (!queue) {
> > /* Try to get a single page. You are my only hope! */
> > @@ -994,21 +985,46 @@ static struct virtqueue *vring_create_virtqueue_split(
> > &dma_addr, GFP_KERNEL|__GFP_ZERO);
> > }
> > if (!queue)
> > - return NULL;
> > + return -ENOMEM;
> > +
> > + vring_init(&vring_split->vring, num, queue, vring_align);
> >
> > - queue_size_in_bytes = vring_size(num, vring_align);
> > - vring_init(&vring, num, queue, vring_align);
> > + vring_split->queue_dma_addr = dma_addr;
> > + vring_split->queue_size_in_bytes = vring_size(num, vring_align);
> > +
> > + return 0;
> > +}
> > +
> > +static struct virtqueue *vring_create_virtqueue_split(
> > + unsigned int index,
> > + unsigned int num,
> > + unsigned int vring_align,
> > + struct virtio_device *vdev,
> > + bool weak_barriers,
> > + bool may_reduce_num,
> > + bool context,
> > + bool (*notify)(struct virtqueue *),
> > + void (*callback)(struct virtqueue *),
> > + const char *name)
> > +{
> > + struct vring_virtqueue_split vring_split = {};
> > + struct virtqueue *vq;
> > + int err;
> > +
> > + err = vring_alloc_queue_split(&vring_split, vdev, num, vring_align,
> > + may_reduce_num);
> > + if (err)
> > + return NULL;
> >
> > - vq = __vring_new_virtqueue(index, vring, vdev, weak_barriers, context,
> > - notify, callback, name);
> > + vq = __vring_new_virtqueue(index, vring_split.vring, vdev, weak_barriers,
> > + context, notify, callback, name);
> > if (!vq) {
> > - vring_free_queue(vdev, queue_size_in_bytes, queue,
> > - dma_addr);
> > + vring_free_split(&vring_split, vdev);
> > return NULL;
> > }
> >
> > - to_vvq(vq)->split.queue_dma_addr = dma_addr;
> > - to_vvq(vq)->split.queue_size_in_bytes = queue_size_in_bytes;
> > + to_vvq(vq)->split.queue_dma_addr = vring_split.queue_dma_addr;
> > + to_vvq(vq)->split.queue_size_in_bytes = vring_split.queue_size_in_bytes;
>
>
> This still seems a little bit redundant since the current logic is a
> little bit complicated since the vq->split is not initialized in a
> single place.
>
> I wonder if it's better to:
>
> vring_alloc_queue_split()
> vring_alloc_desc_extra() (reorder to make patch 9 come first)
>
> then we can simply assign vring_split to vq->split in
> __vring_new_virtqueue() since it has:
>
>     vq->split.queue_dma_addr = 0;
>     vq->split.queue_size_in_bytes = 0;
>
>     vq->split.vring = vring;
>     vq->split.avail_flags_shadow = 0;
>     vq->split.avail_idx_shadow = 0;
>
> This seems to simplify the logic and task of e.g
> virtqueue_vring_attach_split() to a simple:
>
> vq->split= vring_split;

This does look simpler. The reason for not doing this is that the argument
accepted by __vring_new_virtqueue() is "struct vring", and
__vring_new_virtqueue() is an export symbol.

I took a look, and the only external direct call to __vring_new_virtqueue is
here.

tools/virtio/virtio_test.c
static void vq_reset(struct vq_info *info, int num, struct virtio_device *vdev)
{
if (info->vq)
vring_del_virtqueue(info->vq);

memset(info->ring, 0, vring_size(num, 4096));
vring_init(&info->vring, num, info->ring, 4096);
info->vq = __vring_new_virtqueue(info->idx, info->vring, vdev, true,
false, vq_notify, vq_callback, "test");
assert(info->vq);
info->vq->priv = info;
}

I think this could be replaced with vring_new_virtqueue() so that we don't need
to make __vring_new_virtqueue as an export function so we can make some
modifications to it.

nit: vring_alloc_desc_extra() should not have to be extract from
__vring_new_virtqueue() .

Thanks.

>
> And if this makes sense, we can do something similar to packed ring.
>
> Thanks
>
>
> > to_vvq(vq)->we_own_ring = true;
> >
> > return vq;
>