Re: [Qemu-devel] [PATCH 6/8] virtio: flush/push support for packed ring

[Jason Wang <jasowang@redhat.com>]

On 2018年04月04日 20:54, wexu@redhat.com wrote:
> From: Wei Xu <wexu@redhat.com>
>
> Signed-off-by: Wei Xu <wexu@redhat.com>
> ---
>   hw/virtio/virtio.c | 104 +++++++++++++++++++++++++++++++++++++++++++++--------
>   1 file changed, 90 insertions(+), 14 deletions(-)
>
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 95a4681..def07c6 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -26,6 +26,7 @@
>   
>   #define AVAIL_DESC_PACKED(b) ((b) << 7)
>   #define USED_DESC_PACKED(b)  ((b) << 15)
> +#define VIRTQ_F_DESC_USED(w)  (AVAIL_DESC_PACKED(w) | USED_DESC_PACKED(w))
>   
>   /*
>    * The alignment to use between consumer and producer parts of vring.
> @@ -636,19 +637,11 @@ bool virtqueue_rewind(VirtQueue *vq, unsigned int num)
>   }
>   
>   /* Called within rcu_read_lock().  */
> -void virtqueue_fill(VirtQueue *vq, const VirtQueueElement *elem,
> +static void virtqueue_fill_split(VirtQueue *vq, const VirtQueueElement *elem,
>                       unsigned int len, unsigned int idx)
>   {
>       VRingUsedElem uelem;
>   
> -    trace_virtqueue_fill(vq, elem, len, idx);
> -
> -    virtqueue_unmap_sg(vq, elem, len);
> -
> -    if (unlikely(vq->vdev->broken)) {
> -        return;
> -    }
> -
>       if (unlikely(!vq->vring.used)) {
>           return;
>       }
> @@ -660,16 +653,66 @@ void virtqueue_fill(VirtQueue *vq, const VirtQueueElement *elem,
>       vring_used_write(vq, &uelem, idx);
>   }
>   
> -/* Called within rcu_read_lock().  */
> -void virtqueue_flush(VirtQueue *vq, unsigned int count)
> +static void virtqueue_fill_packed(VirtQueue *vq, const VirtQueueElement *elem)
>   {
> -    uint16_t old, new;
> +    uint16_t i, w, head;
> +    VRingMemoryRegionCaches *caches;
> +    VRingDescPacked desc = {
> +        .addr = 0,
> +        .flags = 0,
> +    };
> +
> +    if (unlikely(!vq->packed.desc)) {
> +        return;
> +    }
> +
> +    w = elem->wrap_counter;
> +    caches = vring_get_region_caches(vq);
> +    for (i = 0; i < elem->count; i++) {
> +        head = (elem->index + i) % vq->packed.num;

This looks strange, we should get the location from vq->used_idx.

> +        /* Don't toggle the first one since it is the originally one */
> +        if ((i > 0) && (!head)) {
> +            w ^= 1;
> +        }
> +
> +        desc.id = elem->index;
> +        desc.flags = VIRTQ_F_DESC_USED(w);
> +        desc.len = elem->len[i];

This should be determined by the caller, especially for the WRITE 
descriptor.

> +        virtio_tswap16s(vq->vdev, &desc.id);
> +        virtio_tswap32s(vq->vdev, &desc.len);
> +        virtio_tswap16s(vq->vdev, &desc.flags);
> +        address_space_write_cached(&caches->desc,
> +                                   sizeof(VRingDescPacked) * head, &desc,
> +                                   sizeof(VRingDescPacked));

We should make sure flags is written after other fileds. So need a wmb() 
between the two writes.

> +        address_space_cache_invalidate(&caches->desc,
> +                                       sizeof(VRingDescPacked) * head,
> +                                       sizeof(VRingDescPacked));
> +    }

Have you tested the case when elem->count is greater than one? According 
to the spec device only need to write a single used descriptor for the 
whole list.

> +}
> +
> +void virtqueue_fill(VirtQueue *vq, const VirtQueueElement *elem,
> +                    unsigned int len, unsigned int idx)
> +{
> +    trace_virtqueue_fill(vq, elem, len, idx);
> +
> +    virtqueue_unmap_sg(vq, elem, len);
>   
>       if (unlikely(vq->vdev->broken)) {
> -        vq->inuse -= count;
>           return;
>       }
>   
> +    if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED)) {
> +        virtqueue_fill_packed(vq, elem);
> +    } else {
> +        virtqueue_fill_split(vq, elem, len, idx);
> +    }
> +}
> +
> +/* Called within rcu_read_lock().  */
> +static void virtqueue_flush_split(VirtQueue *vq, unsigned int count)
> +{
> +    uint16_t old, new;
> +
>       if (unlikely(!vq->vring.used)) {
>           return;
>       }
> @@ -685,12 +728,45 @@ void virtqueue_flush(VirtQueue *vq, unsigned int count)
>           vq->signalled_used_valid = false;
>   }
>   
> +static void virtqueue_flush_packed(VirtQueue *vq, unsigned int count)
> +{
> +    if (unlikely(!vq->packed.desc)) {
> +        return;
> +    }
> +
> +    vq->inuse -= count;
> +
> +    /* FIXME: is this correct? */
> +    if (vq->inuse) {
> +        return;
> +    }

I think the semantics here is write flags here.

> +}
> +
> +void virtqueue_flush(VirtQueue *vq, unsigned int count)
> +{
> +    if (unlikely(vq->vdev->broken)) {
> +        vq->inuse -= count;
> +        return;
> +    }
> +
> +    if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED)) {
> +        virtqueue_flush_packed(vq, count);
> +    } else {
> +        virtqueue_flush_split(vq, count);
> +    }
> +}
> +
>   void virtqueue_push(VirtQueue *vq, const VirtQueueElement *elem,
>                       unsigned int len)
>   {
>       rcu_read_lock();
>       virtqueue_fill(vq, elem, len, 0);
> -    virtqueue_flush(vq, 1);
> +    if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED)) {
> +        /* FIXME: How to deal with the length field for chained desc */
> +        virtqueue_flush(vq, elem->count);
> +    } else {
> +        virtqueue_flush(vq, 1);
> +    }
>       rcu_read_unlock();
>   }
>   

Thanks