linux-kernel - Re: [PATCHv2] vhost-net: utilize PUBLISH_USED

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Date:	Tue, 18 May 2010 13:31:29 +0200
From:	Juan Quintela <quintela@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	davem@...emloft.net, Rusty Russell <rusty@...tcorp.com.au>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Arnd Bergmann <arnd@...db.de>, kvm@...r.kernel.org,
	virtualization@...ts.osdl.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, alex.williamson@...hat.com,
	amit.shah@...hat.com
Subject: Re: [PATCHv2] vhost-net: utilize PUBLISH_USED_IDX feature

"Michael S. Tsirkin" <mst@...hat.com> wrote:
> With PUBLISH_USED_IDX, guest tells us which used entries
> it has consumed. This can be used to reduce the number
> of interrupts: after we write a used entry, if the guest has not yet
> consumed the previous entry, or if the guest has already consumed the
> new entry, we do not need to interrupt.
> This imporves bandwidth by 30% under some workflows.
>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
>
> This is on top of Rusty's tree and depends on the virtio patch.
>
> Changes from v1:
> fix build

Minor nit if you have to respin it:

>  /* This actually signals the guest, using eventfd. */
> -void vhost_signal(struct vhost_dev *dev, struct vhost_virtqueue *vq)
> +static void vhost_signal(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>  {
>  	__u16 flags;
> +	__u16 used;

local var named "used" here

>  	/* Flush out used index updates. This is paired
>  	 * with the barrier that the Guest executes when enabling
>  	 * interrupts. */
> @@ -1053,6 +1057,17 @@ void vhost_signal(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>  	     !vhost_has_feature(dev, VIRTIO_F_NOTIFY_ON_EMPTY)))
>  		return;
>  
> +	if (vhost_has_feature(dev, VIRTIO_RING_F_PUBLISH_USED)) {
> +		__u16 used;

and a "more" local one also named "used" :)

I guess you want to remove the previous declaration, as var is only used
in this block.

> +		if (get_user(used, vq->last_used)) {
> +			vq_err(vq, "Failed to get last used idx");
> +			return;
> +		}
> +
> +		if (used != (u16)(vq->last_used_idx - 1))
> +			return;
> +	}
> +
>  	/* Signal the Guest tell them we used something up. */
>  	if (vq->call_ctx)
>  		eventfd_signal(vq->call_ctx, 1);

Rest of patch looks ok, and as a bonus un-export two functions.

Later, Juan.
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