linux-kernel - Re: [PATCH RFC 2/2] vhost: packed ring support

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Date:   Tue, 27 Feb 2018 17:03:20 +0800
From:   Tiwei Bie <tiwei.bie@...el.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     mst@...hat.com, virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        wexu@...hat.com, jfreimann@...hat.com
Subject: Re: [PATCH RFC 2/2] vhost: packed ring support

On Wed, Feb 14, 2018 at 10:37:09AM +0800, Jason Wang wrote:
[...]
> +static void set_desc_used(struct vhost_virtqueue *vq,
> +			  struct vring_desc_packed *desc, bool wrap_counter)
> +{
> +	__virtio16 flags = desc->flags;
> +
> +	if (wrap_counter) {
> +		desc->flags |= cpu_to_vhost16(vq, DESC_AVAIL);
> +		desc->flags |= cpu_to_vhost16(vq, DESC_USED);
> +	} else {
> +		desc->flags &= ~cpu_to_vhost16(vq, DESC_AVAIL);
> +		desc->flags &= ~cpu_to_vhost16(vq, DESC_USED);
> +	}
> +
> +	desc->flags = flags;

The `desc->flags` is restored after the change.

> +}
> +
> +static int vhost_get_vq_desc_packed(struct vhost_virtqueue *vq,
> +				    struct iovec iov[], unsigned int iov_size,
> +				    unsigned int *out_num, unsigned int *in_num,
> +				    struct vhost_log *log,
> +				    unsigned int *log_num)
> +{
> +	struct vring_desc_packed desc;
> +	int ret, access, i;
> +	u16 avail_idx = vq->last_avail_idx;
> +
> +	/* When we start there are none of either input nor output. */
> +	*out_num = *in_num = 0;
> +	if (unlikely(log))
> +		*log_num = 0;
> +
> +	do {
> +		unsigned int iov_count = *in_num + *out_num;
> +
> +		i = vq->last_avail_idx & (vq->num - 1);

The queue size may not be a power of 2 in packed ring.

Best regards,
Tiwei Bie