Re: [PATCH net-next v2 3/5] virtio_ring: add packed ring support

[Jason Wang <jasowang@redhat.com>]

On 2018/11/8 上午9:38, Tiwei Bie wrote:
>>> +
>>> +	if (vq->vq.num_free < descs_used) {
>>> +		pr_debug("Can't add buf len %i - avail = %i\n",
>>> +			 descs_used, vq->vq.num_free);
>>> +		/* FIXME: for historical reasons, we force a notify here if
>>> +		 * there are outgoing parts to the buffer.  Presumably the
>>> +		 * host should service the ring ASAP. */
>> I don't think we have a reason to do this for packed ring.
>> No historical baggage there, right?
> Based on the original commit log, it seems that the notify here
> is just an "optimization". But I don't quite understand what does
> the "the heuristics which KVM uses" refer to. If it's safe to drop
> this in packed ring, I'd like to do it.


According to the commit log, it seems like a workaround of lguest 
networking backend. I agree to drop it, we should not have such burden.

But we should notice that, with this removed, the compare between packed 
vs split is kind of unfair. Consider the removal of lguest support 
recently, maybe we can drop this for split ring as well?

Thanks


>
> commit 44653eae1407f79dff6f52fcf594ae84cb165ec4
> Author: Rusty Russell<rusty@rustcorp.com.au>
> Date:   Fri Jul 25 12:06:04 2008 -0500
>
>      virtio: don't always force a notification when ring is full
>      
>      We force notification when the ring is full, even if the host has
>      indicated it doesn't want to know.  This seemed like a good idea at
>      the time: if we fill the transmit ring, we should tell the host
>      immediately.
>      
>      Unfortunately this logic also applies to the receiving ring, which is
>      refilled constantly.  We should introduce real notification thesholds
>      to replace this logic.  Meanwhile, removing the logic altogether breaks
>      the heuristics which KVM uses, so we use a hack: only notify if there are
>      outgoing parts of the new buffer.
>      
>      Here are the number of exits with lguest's crappy network implementation:
>      Before:
>              network xmit 7859051 recv 236420
>      After:
>              network xmit 7858610 recv 118136
>      
>      Signed-off-by: Rusty Russell<rusty@rustcorp.com.au>
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 72bf8bc09014..21d9a62767af 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -87,8 +87,11 @@ static int vring_add_buf(struct virtqueue *_vq,
>   	if (vq->num_free < out + in) {
>   		pr_debug("Can't add buf len %i - avail = %i\n",
>   			 out + in, vq->num_free);
> -		/* We notify*even if*  VRING_USED_F_NO_NOTIFY is set here. */
> -		vq->notify(&vq->vq);
> +		/* FIXME: for historical reasons, we force a notify here if
> +		 * there are outgoing parts to the buffer.  Presumably the
> +		 * host should service the ring ASAP. */
> +		if (out)
> +			vq->notify(&vq->vq);
>   		END_USE(vq);
>   		return -ENOSPC;
>   	}
>
>