Re: [PATCH 1/2] virtio: allow to detach a buffer from the virtqueue

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Dec 20, 2017 at 06:46:42PM +0100, Greg Kurz wrote:
> It is possible for a device to stop using buffers without pushing them
> back to the driver. This is the case for example with the 9p virtio
> device: if the driver flushes an in-flight request, the 9p specification
> specification [*] mandates the server to "to purge the pending response".
> The reply to the flush request indicates that the 9p server has stopped
> using the buffers of the flushed in-flight request. But since the server
> doesn't push back the associated buffers, they don't go back to the free
> list. This leads the virtqueue to end up with a single slot to handle all
> the dialog with the device, ie, serialize all I/Os.
> 
> This patch hence gives the possibility for device specific code to
> explicitly detach a given buffer from the used ring and put it back
> to the free list.
> 
> [*] http://man.cat-v.org/plan_9/5/flush
> 
> Signed-off-by: Greg Kurz <groug@kaod.org>

It would be better to just change the server to mark all flushed
requests as used. Why isn't that an option?

> ---
>  drivers/virtio/virtio_ring.c |   28 ++++++++++++++++++++++++++++
>  include/linux/virtio.h       |    1 +
>  2 files changed, 29 insertions(+)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index eb30f3e09a47..886e9d054de3 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -936,6 +936,34 @@ void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_detach_unused_buf);
>  
> +/**
> + * virtqueue_detach_buf - detach specific buffer
> + * @vq: the struct virtqueue we're talking about.
> + *
> + * Returns NULL or the "data" token handed to virtqueue_add_*().
> + * This should only be used if the driver really knows the buffer
> + * isn't needed anymore by the device.
> + */
> +void *virtqueue_detach_buf(struct virtqueue *_vq, void *buf)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	unsigned int i;
> +
> +	START_USE(vq);
> +
> +	for (i = 0; i < vq->vring.num; i++) {
> +		if (vq->desc_state[i].data != buf)
> +			continue;
> +		detach_buf(vq, i, NULL);
> +		END_USE(vq);
> +		return buf;
> +	}
> +
> +	END_USE(vq);
> +	return NULL;
> +}
> +EXPORT_SYMBOL_GPL(virtqueue_detach_buf);
> +
>  irqreturn_t vring_interrupt(int irq, void *_vq)
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);

Unfortunately the used index gets out of sync with the available index
then.

E.g. you are breaking the invariant that used == avail means ring empty.

Any chance to preserve this invariant?



> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index 988c7355bc22..850158518ce5 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -80,6 +80,7 @@ bool virtqueue_poll(struct virtqueue *vq, unsigned);
>  bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
>  
>  void *virtqueue_detach_unused_buf(struct virtqueue *vq);
> +void *virtqueue_detach_buf(struct virtqueue *vq, void *buf);
>  
>  unsigned int virtqueue_get_vring_size(struct virtqueue *vq);
>