linux-kernel - Re: [PATCH] virtio: fix size computation according to the definition of struct vring_used in vring

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Date:	Sun, 28 Aug 2011 14:31:32 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Wang Sheng-Hui <shhuiw@...il.com>
Cc:	wanlong.gao@...il.com, rusty@...tcorp.com.au,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] virtio: fix size computation according to the definition
 of struct vring_used in vring_size

On Sat, Aug 27, 2011 at 05:52:02PM +0800, Wang Sheng-Hui wrote:
> On 2011年08月27日 17:34, Wang Sheng-Hui wrote:
> > On 2011年08月27日 10:49, Wanlong Gao wrote:
> >> On Sat, 2011-08-27 at 09:07 +0800, Wang Sheng-Hui wrote:
> >>> The patch is against 3.1-rc3.
> >>>
> >>> struct vring_used has two __u16 fields plus array of struct vring_used_elem.
> >>> Current vring_size counts the __u16 fields to 3. Fix it to 2 in the patch.
> >>>
> >>> Signed-off-by: Wang Sheng-Hui <shhuiw@...il.com>
> >>> ---
> >>>  include/linux/virtio_ring.h |    2 +-
> >>>  1 files changed, 1 insertions(+), 1 deletions(-)
> >>>
> >>> diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
> >>> index 4a32cb6..fcda152 100644
> >>> --- a/include/linux/virtio_ring.h
> >>> +++ b/include/linux/virtio_ring.h
> >>> @@ -143,7 +143,7 @@ static inline unsigned vring_size(unsigned int num, unsigned long align)
> >>>  {
> >>>  	return ((sizeof(struct vring_desc) * num + sizeof(__u16) * (2 + num)
> >>>  		 + align - 1) & ~(align - 1))
> >>> -		+ sizeof(__u16) * 3 + sizeof(struct vring_used_elem) * num;
> >>> +		+ sizeof(__u16) * 2 + sizeof(struct vring_used_elem) * num;
> >>>  }
> >>>  
> >>>  /* The following is used with USED_EVENT_IDX and AVAIL_EVENT_IDX */
> >>
> >> Hi:
> >> I'm not deep into it, but I think you can see this:
> >> http://marc.info/?l=git-commits-head&m=130687915816130&w=2
> >>
> >> Thanks
> >> -Wanlong Gao
> >>
> > 
> > Thanks.
> > 
> > I checked the comments, and think we should patch vring_init
> > and vring_size according to the layout description in the comments.
> > Right?
> 
> New patch generated. Please check it. Thanks,


> [PATCH] virtio: modify vring_init and vring_size to take account of the layout containing *_event_idx
> 
> The patch is against 3.1-rc3.
> 
> Based on the layout description in the comments, take account of
> the *_event_idx in functions vring_init and vring_size.
> 
> Signed-off-by: Wang Sheng-Hui <shhuiw@...il.com>

Is this a bugfix or just a cleanup?
Alignment makes us get the same values with and without, right?

> ---
>  include/linux/virtio_ring.h |    6 +++---
>  1 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
> index 4a32cb6..300af76 100644
> --- a/include/linux/virtio_ring.h
> +++ b/include/linux/virtio_ring.h
> @@ -135,13 +135,13 @@ static inline void vring_init(struct vring *vr, unsigned int num, void *p,
>  	vr->num = num;
>  	vr->desc = p;
>  	vr->avail = p + num*sizeof(struct vring_desc);
> -	vr->used = (void *)(((unsigned long)&vr->avail->ring[num] + align-1)
> -			    & ~(align - 1));
> +	vr->used = (void *)(((unsigned long)&vr->avail->ring[num] + 16
> +		+ align-1) & ~(align - 1));
>  }
>  
>  static inline unsigned vring_size(unsigned int num, unsigned long align)
>  {
> -	return ((sizeof(struct vring_desc) * num + sizeof(__u16) * (2 + num)
> +	return ((sizeof(struct vring_desc) * num + sizeof(__u16) * (3 + num)
>  		 + align - 1) & ~(align - 1))
>  		+ sizeof(__u16) * 3 + sizeof(struct vring_used_elem) * num;
>  }
> -- 
> 1.7.1
> 
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