netdev - Re: [PATCH 09/18] virtio: use avail

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Date:	Mon, 09 May 2011 14:03:26 +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: [PATCH 09/18] virtio: use avail_event index

On Wed, 4 May 2011 23:51:47 +0300, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> Use the new avail_event feature to reduce the number
> of exits from the guest.

Figures here would be nice :)

> @@ -228,6 +237,12 @@ add_head:
>  	 * new available array entries. */
>  	virtio_wmb();
>  	vq->vring.avail->idx++;
> +	/* If the driver never bothers to kick in a very long while,
> +	 * avail index might wrap around. If that happens, invalidate
> +	 * kicked_avail index we stored. TODO: make sure all drivers
> +	 * kick at least once in 2^16 and remove this. */
> +	if (unlikely(vq->vring.avail->idx == vq->kicked_avail))
> +		vq->kicked_avail_valid = true;

If they don't, they're already buggy.  Simply do:
        WARN_ON(vq->vring.avail->idx == vq->kicked_avail);

> +static bool vring_notify(struct vring_virtqueue *vq)
> +{
> +	u16 old, new;
> +	bool v;
> +	if (!vq->event)
> +		return !(vq->vring.used->flags & VRING_USED_F_NO_NOTIFY);
> +
> +	v = vq->kicked_avail_valid;
> +	old = vq->kicked_avail;
> +	new = vq->kicked_avail = vq->vring.avail->idx;
> +	vq->kicked_avail_valid = true;
> +	if (unlikely(!v))
> +		return true;

This is the only place you actually used kicked_avail_valid.  Is it
possible to initialize it in such a way that you can remove this?

> @@ -482,6 +517,8 @@ void vring_transport_features(struct virtio_device *vdev)
>  			break;
>  		case VIRTIO_RING_F_USED_EVENT_IDX:
>  			break;
> +		case VIRTIO_RING_F_AVAIL_EVENT_IDX:
> +			break;
>  		default:
>  			/* We don't understand this bit. */
>  			clear_bit(i, vdev->features);

Does this belong in a prior patch?

Thanks,
Rusty.
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