Subject: Re: [RFC 2/3] virtio_ring: add VIRTIO_RING_NO_LEGACY

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 7 Dec 2018 13:05:35 -0500

On Fri, Dec 07, 2018 at 04:48:41PM +0800, Tiwei Bie wrote:
> Introduce VIRTIO_RING_NO_LEGACY to support disabling legacy
> macros and layout definitions.
> 
> Suggested-by: Michael S. Tsirkin <[email-masked]>
> Signed-off-by: Tiwei Bie <[email-masked]>
> ---
> VRING_AVAIL_ALIGN_SIZE, VRING_USED_ALIGN_SIZE and VRING_DESC_ALIGN_SIZE
> are not pre-virtio 1.0, but can also be disabled by VIRTIO_RING_NO_LEGACY
> in this patch, because their names are not consistent with other names.
> Not sure whether this is a good idea. If we want this, we may also want
> to define _SPLIT_ version for them.

I don't think it's a good idea to have alignment in there - the point of
NO_LEGACY is to help catch bugs not to sanitize coding style IMHO.

And spec calls "legacy" the 0.X interfaces, let's not muddy the waters.

> 
>  include/uapi/linux/virtio_ring.h | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/virtio_ring.h
> index 9b0c0d92ab62..192573827850 100644
> --- a/include/uapi/linux/virtio_ring.h
> +++ b/include/uapi/linux/virtio_ring.h
> @@ -37,6 +37,7 @@
>  #include <linux/types.h>
>  #include <linux/virtio_types.h>
>  
> +#ifndef VIRTIO_RING_NO_LEGACY
>  /*
>   * Notice: unlike other _F_ flags, below flags are defined as shifted
>   * values instead of shifts for compatibility.
> @@ -51,6 +52,7 @@
>  #define VRING_USED_F_NO_NOTIFY	1
>  /* Same as VRING_SPLIT_AVAIL_F_NO_INTERRUPT. */
>  #define VRING_AVAIL_F_NO_INTERRUPT	1
> +#endif /* VIRTIO_RING_NO_LEGACY */
>  
>  /* Mark a buffer as continuing via the next field in split ring. */
>  #define VRING_SPLIT_DESC_F_NEXT		0
> @@ -151,6 +153,7 @@ struct vring {
>  	struct vring_used *used;
>  };
>  
> +#ifndef VIRTIO_RING_NO_LEGACY
>  /* Alignment requirements for vring elements.
>   * When using pre-virtio 1.0 layout, these fall out naturally.
>   */
> @@ -203,6 +206,7 @@ static inline unsigned vring_size(unsigned int num, unsigned long align)
>  		 + align - 1) & ~(align - 1))
>  		+ sizeof(__virtio16) * 3 + sizeof(struct vring_used_elem) * num;
>  }
> +#endif /* VIRTIO_RING_NO_LEGACY */
>  
>  /* The following is used with USED_EVENT_IDX and AVAIL_EVENT_IDX */
>  /* Assuming a given event_idx value from the other side, if
> -- 
> 2.17.1