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

[Tiwei Bie]

On Fri, Dec 07, 2018 at 01:05:35PM -0500, Michael S. Tsirkin wrote:
> 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 <mst@xxxxxxxxxx>
> > Signed-off-by: Tiwei Bie <tiwei.bie@xxxxxxxxx>
> > ---
> > 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.

Make sense. Thanks!

> 
> > 
> >  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