Subject: Re: [RFC v2] virtio: support packed ring

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 13 Apr 2018 18:22:45 +0300

On Sun, Apr 01, 2018 at 10:12:16PM +0800, Tiwei Bie wrote:
> +static inline bool more_used(const struct vring_virtqueue *vq)
> +{
> +	return vq->packed ? more_used_packed(vq) : more_used_split(vq);
> +}
> +
> +void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq, unsigned int *len,
> +				  void **ctx)
> +{
> +	struct vring_virtqueue *vq = to_vvq(_vq);
> +	void *ret;
> +	unsigned int i;
> +	u16 last_used;
> +
> +	START_USE(vq);
> +
> +	if (unlikely(vq->broken)) {
> +		END_USE(vq);
> +		return NULL;
> +	}
> +
> +	if (!more_used(vq)) {
> +		pr_debug("No more buffers in queue\n");
> +		END_USE(vq);
> +		return NULL;
> +	}

So virtqueue_get_buf_ctx_split should only call more_used_split.

to avoid such issues I think we should lay out the code like this:

XXX_split

XXX_packed

XXX wrappers

> +/* The standard layout

I'd drop standard here.

> for the packed ring is a continuous chunk of memory
> + * which looks like this.
> + *
> + * struct vring_packed
> + * {

Can the opening bracket go on the prev line pls?

> + *	// The actual descriptors (16 bytes each)
> + *	struct vring_packed_desc desc[num];
> + *
> + *	// Padding to the next align boundary.
> + *	char pad[];
> + *
> + *	// Driver Event Suppression
> + *	struct vring_packed_desc_event driver;
> + *
> + *	// Device Event Suppression
> + *	struct vring_packed_desc_event device;

Maybe that's how our driver does it but it's not based on spec
so I don't think this belongs in the header.

> + * };
> + */
> +
> +static inline unsigned vring_packed_size(unsigned int num, unsigned long align)
> +{
> +	return ((sizeof(struct vring_packed_desc) * num + align - 1)
> +		& ~(align - 1)) + sizeof(struct vring_packed_desc_event) * 2;
> +}
> +

Cant say this API makes sense for me.

>  #endif /* _UAPI_LINUX_VIRTIO_RING_H */
> -- 
> 2.11.0