linux-kernel - Re: [PATCH 08/18] virtio_ring: support for used

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Date:	Mon, 09 May 2011 13:47:32 +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: [PATCH 08/18] virtio_ring: support for used_event idx feature

On Wed, 4 May 2011 23:51:38 +0300, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> Add support for the used_event idx feature: when enabling
> interrupts, publish the current avail index value to
> the host so that we get interrupts on the next update.
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
>  drivers/virtio/virtio_ring.c |   14 ++++++++++++++
>  1 files changed, 14 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 507d6eb..3a3ed75 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -320,6 +320,14 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
>  	ret = vq->data[i];
>  	detach_buf(vq, i);
>  	vq->last_used_idx++;
> +	/* If we expect an interrupt for the next entry, tell host
> +	 * by writing event index and flush out the write before
> +	 * the read in the next get_buf call. */
> +	if (!(vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT)) {
> +		vring_used_event(&vq->vring) = vq->last_used_idx;
> +		virtio_mb();
> +	}
> +

Hmm, so you're still using the avail->flags; it's just if thresholding
is enabled the host will ignore it?

It's a little subtle, but it keeps this patch small.  Perhaps we'll want
to make it more explicit later.

Thanks,
Rusty.
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