Re: [Xen-devel] [PATCH net-next] xen-netback: fix xenvif_count_skb_slots()

[Matt Wilson <msw@xxxxxxxxx>]

On Sun, Oct 06, 2013 at 05:06:52PM -0700, Matt Wilson wrote:
> On Fri, Oct 04, 2013 at 05:26:23PM +0100, Paul Durrant wrote:
> > Commit 4f0581d25827d5e864bcf07b05d73d0d12a20a5c introduced an error into
> > xenvif_count_skb_slots() for skbs with a linear area spanning a page
> > boundary. The alignment of skb->data needs to be taken into account, not
> > just the head length. This patch fixes the issue by dry-running the code
> > from xenvif_gop_skb() (and adjusting the comment above the function to note
> > that).
> > 
> > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> > Cc: Xi Xiong <xixiong@xxxxxxxxxx>
> > Cc: Matt Wilson <msw@xxxxxxxxxx>
> > Cc: Annie Li <annie.li@xxxxxxxxxx>
> > Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> > Cc: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
> 
> Paul, can you reconcile this change with the one made by Simon in cs
> 8f985b4f7a5394c8f8725a5109451a541ddb9eea?

Correction: e26b203ede31fffd52571a5ba607a26c79dc5c0d

> --msw
> 
> > ---
> >  drivers/net/xen-netback/netback.c |   17 +++++++++++++++--
> >  1 file changed, 15 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/net/xen-netback/netback.c 
> > b/drivers/net/xen-netback/netback.c
> > index d0b0feb..6f680f4 100644
> > --- a/drivers/net/xen-netback/netback.c
> > +++ b/drivers/net/xen-netback/netback.c
> > @@ -223,15 +223,28 @@ static bool start_new_rx_buffer(int offset, unsigned 
> > long size, int head)
> >  /*
> >   * Figure out how many ring slots we're going to need to send @skb to
> >   * the guest. This function is essentially a dry run of
> > - * xenvif_gop_frag_copy.
> > + * xenvif_gop_skb.
> >   */
> >  unsigned int xenvif_count_skb_slots(struct xenvif *vif, struct sk_buff 
> > *skb)
> >  {
> > +   unsigned char *data;
> >     unsigned int count;
> >     int i, copy_off;
> >     struct skb_cb_overlay *sco;
> >  
> > -   count = DIV_ROUND_UP(skb_headlen(skb), PAGE_SIZE);
> > +   count = 0;
> > +
> > +   data = skb->data;
> > +   while (data < skb_tail_pointer(skb)) {
> > +           unsigned int offset = offset_in_page(data);
> > +           unsigned int len = PAGE_SIZE - offset;
> > +
> > +           if (data + len > skb_tail_pointer(skb))
> > +                   len = skb_tail_pointer(skb) - data;
> > +
> > +           count++;
> > +           data += len;
> > +   }
> >  
> >     copy_off = skb_headlen(skb) % PAGE_SIZE;
> >  

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