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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
> Sent: 07 October 2013 11:02
> To: Paul Durrant
> Cc: xen-devel@xxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; Xi Xiong; Matt Wilson;
> Annie Li; Wei Liu; Ian Campbell
> Subject: Re: [PATCH net-next] xen-netback: fix xenvif_count_skb_slots()
> 
> 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).
> >
> 
> If I'm not mistaken the change in commit 4f0581d2 is correct because we
> changed the way that the ring is packed. Now you seem to fall back to
> the original scheme (or something in between without reverting later
> other changes in that commit).
> 

It's not possible to use a single grant copy to copy to even a 2-byte linear 
area that spans a page boundary so you have to take into account the alignment 
of skb->data. How the ring is packed is not relevant.

> Do you have instruction to reproduce the bug? Can you paste some
> detailed oops message?
> 

I don't have the message to hand, but it's this BUG_ON that I hit:

        BUG_ON(npo.copy_prod > ARRAY_SIZE(vif->grant_copy_op));

I.e. we blow the grant copy op array.

  Paul

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