Re: [Qemu-devel] [PATCH 1/3] virtio: add virtio_detach_element()

[Ladi Prosek]

On Mon, Sep 19, 2016 at 3:28 PM, Stefan Hajnoczi <address@hidden> wrote:
> During device reset or similar situations a VirtQueueElement needs to be
> freed without pushing it onto the used ring or rewinding the virtqueue.
> Extract a new function to do this.
>
> Later patches add virtio_detach_element() calls to existing device so
> that scatter-gather lists are unmapped and vq->inuse goes back to zero
> during device reset.  Currently some devices don't bother and simply
> call g_free(elem) which is not a clean way to throw away a
> VirtQueueElement.
>
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
>  hw/virtio/virtio.c         | 27 +++++++++++++++++++++++++--
>  include/hw/virtio/virtio.h |  2 ++
>  2 files changed, 27 insertions(+), 2 deletions(-)
>
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index fcf3358..adcef45 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -264,12 +264,35 @@ static void virtqueue_unmap_sg(VirtQueue *vq, const 
> VirtQueueElement *elem,
>                                    0, elem->out_sg[i].iov_len);
>  }
>
> +/* virtqueue_detach_element:
> + * @vq: The #VirtQueue
> + * @elem: The #VirtQueueElement
> + * @len: number of bytes written
> + *
> + * Detach the element from the virtqueue.  This function is suitable for 
> device
> + * reset or other situations where a #VirtQueueElement is simply freed and 
> will
> + * not be pushed or discarded.
> + */
> +void virtqueue_detach_element(VirtQueue *vq, const VirtQueueElement *elem,
> +                              unsigned int len)
> +{
> +    vq->inuse--;
> +    virtqueue_unmap_sg(vq, elem, len);
> +}
> +
> +/* virtqueue_discard:
> + * @vq: The #VirtQueue
> + * @elem: The #VirtQueueElement
> + * @len: number of bytes written
> + *
> + * Pretend the most recent element wasn't popped from the virtqueue.  The 
> next
> + * call to virtqueue_pop() will refetch the element.
> + */
>  void virtqueue_discard(VirtQueue *vq, const VirtQueueElement *elem,
>                         unsigned int len)
>  {
>      vq->last_avail_idx--;
> -    vq->inuse--;
> -    virtqueue_unmap_sg(vq, elem, len);
> +    virtqueue_detach_element(vq, elem, len);

Random comment, not directly related to this change. Would it be worth
adding an assert to this function that elem->index and
vq->last_avail_idx match? In other words, enforce the "most recent"
qualifier mentioned in the comment. As more virtqueue_* functions are
added and the complexity goes up, it is easy to get confused. Also, I
think that naming this function virtqueue_unpop instead of
virtqueue_discard would help.

>  }
>
>  /* virtqueue_rewind:
> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> index f05559d..ad1e2d6 100644
> --- a/include/hw/virtio/virtio.h
> +++ b/include/hw/virtio/virtio.h
> @@ -152,6 +152,8 @@ void *virtqueue_alloc_element(size_t sz, unsigned 
> out_num, unsigned in_num);
>  void virtqueue_push(VirtQueue *vq, const VirtQueueElement *elem,
>                      unsigned int len);
>  void virtqueue_flush(VirtQueue *vq, unsigned int count);
> +void virtqueue_detach_element(VirtQueue *vq, const VirtQueueElement *elem,
> +                              unsigned int len);
>  void virtqueue_discard(VirtQueue *vq, const VirtQueueElement *elem,
>                         unsigned int len);
>  bool virtqueue_rewind(VirtQueue *vq, unsigned int num);
> --
> 2.7.4
>

Reviewed-by: Ladi Prosek <address@hidden>

Thanks!
Ladi