linux-kernel - Re: [PATCHv2 11/14] virtio: don't delay avail index update

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Date:	Sat, 21 May 2011 11:56:32 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org
Cc:	Carsten Otte <cotte@...ibm.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	linux390@...ibm.com, Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Shirley Ma <xma@...ibm.com>, lguest@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	linux-s390@...r.kernel.org, kvm@...r.kernel.org,
	Krishna Kumar <krkumar2@...ibm.com>,
	Tom Lendacky <tahm@...ux.vnet.ibm.com>, steved@...ibm.com,
	habanero@...ux.vnet.ibm.com
Subject: Re: [PATCHv2 11/14] virtio: don't delay avail index update

On Fri, 20 May 2011 02:12:19 +0300, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> Update avail index immediately instead of upon kick:
> for virtio-net RX this helps parallelism with the host.
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
>  drivers/virtio/virtio_ring.c |   28 +++++++++++++++++++---------
>  1 files changed, 19 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index eed5f29..8218fe6 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -89,7 +89,7 @@ struct vring_virtqueue
>  	unsigned int num_free;
>  	/* Head of free buffer list. */
>  	unsigned int free_head;
> -	/* Number we've added since last sync. */
> +	/* Number we've added since last kick. */
>  	unsigned int num_added;

I always like to see obsolescent nomenclature cleaned up like this.
Thanks.

>  	/* Last used index we've seen. */
> @@ -174,6 +174,13 @@ int virtqueue_add_buf_gfp(struct virtqueue *_vq,
>  
>  	BUG_ON(data == NULL);
>  
> +	/* Prevent drivers from adding more than num bufs without a kick. */
> +	if (vq->num_added == vq->vring.num) {
> +		printk(KERN_ERR "gaaa!!!\n");
> +		END_USE(vq);
> +		return -ENOSPC;
> +	}
> +

I like "gaaa!" but it won't tell us which driver.  How about the more
conventional:

        if (WARN_ON(vq->num_added >= vq->vring.num)) {
        	END_USE(vq);
        	return -ENOSPC;
        }

I'd really like to see the results of this patch.  It's useless for
outgoing net traffic (we deal with one packet at a time) but perhaps a
flood of incoming packets would show something.

Thanks,
Rusty.
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