Re: [RFC v2 1/7] vhost: expose used buffers

From: Jason Wang <jasowang@redhat.com>
To: Guo Zhi <qtxuning1999@sjtu.edu.cn>,
	eperezma@redhat.com, sgarzare@redhat.com, mst@redhat.com
Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, virtualization@lists.linux-foundation.org
Subject: Re: [RFC v2 1/7] vhost: expose used buffers
Date: Thu, 25 Aug 2022 15:01:31 +0800	[thread overview]
Message-ID: <3a184162-afdc-9103-e786-66d796389e3a@redhat.com> (raw)
In-Reply-To: <20220817135718.2553-2-qtxuning1999@sjtu.edu.cn>

在 2022/8/17 21:57, Guo Zhi 写道:
> Follow VIRTIO 1.1 spec, only writing out a single used ring for a batch
> of descriptors.
>
> Signed-off-by: Guo Zhi <qtxuning1999@sjtu.edu.cn>
> ---
>   drivers/vhost/vhost.c | 14 ++++++++++++--
>   drivers/vhost/vhost.h |  1 +
>   2 files changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 40097826cff0..7b20fa5a46c3 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2376,10 +2376,20 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
>   	vring_used_elem_t __user *used;
>   	u16 old, new;
>   	int start;
> +	int copy_n = count;
>   
> +	/**
> +	 * If in order feature negotiated, devices can notify the use of a batch of buffers to
> +	 * the driver by only writing out a single used ring entry with the id corresponding
> +	 * to the head entry of the descriptor chain describing the last buffer in the batch.
> +	 */
> +	if (vhost_has_feature(vq, VIRTIO_F_IN_ORDER)) {
> +		copy_n = 1;
> +		heads = &heads[count - 1];

Do we need to check whether or not the buffer is fully used before doing 
this?

> +	}
>   	start = vq->last_used_idx & (vq->num - 1);
>   	used = vq->used->ring + start;
> -	if (vhost_put_used(vq, heads, start, count)) {
> +	if (vhost_put_used(vq, heads, start, copy_n)) {
>   		vq_err(vq, "Failed to write used");
>   		return -EFAULT;
>   	}
> @@ -2410,7 +2420,7 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
>   
>   	start = vq->last_used_idx & (vq->num - 1);
>   	n = vq->num - start;
> -	if (n < count) {
> +	if (n < count && !vhost_has_feature(vq, VIRTIO_F_IN_ORDER)) {
>   		r = __vhost_add_used_n(vq, heads, n);
>   		if (r < 0)
>   			return r;
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index d9109107af08..0d5c49a30421 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -236,6 +236,7 @@ enum {
>   	VHOST_FEATURES = (1ULL << VIRTIO_F_NOTIFY_ON_EMPTY) |
>   			 (1ULL << VIRTIO_RING_F_INDIRECT_DESC) |
>   			 (1ULL << VIRTIO_RING_F_EVENT_IDX) |
> +			 (1ULL << VIRTIO_F_IN_ORDER) |
>   			 (1ULL << VHOST_F_LOG_ALL) |

Are we sure all vhost devices can support in-order (especially the SCSI)?

It looks better to start from a device specific one.

Thanks

>   			 (1ULL << VIRTIO_F_ANY_LAYOUT) |
>   			 (1ULL << VIRTIO_F_VERSION_1)