Re: [Xen-devel] [PATCH 5/6] xen-netback: coalesce slots before copying

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Mar 26, 2013 at 11:13:38AM +0000, David Vrabel wrote:
> >>
> >> Separately, it may be sensible for the backend to drop packets with more
> >> frags than max-slots-per-frame up to some threshold where anything more
> >> is considered malicious (i.e., 1 - 18 slots is a valid packet, 19-20 are
> >> dropped and 21 or more is a fatal error).
> >>
> > 
> > Why drop the packet when we are able to process it? Frontend cannot know
> > it has crossed the line anyway.
> 
> Because it's a change to the protocol and we do not want to do this for
> a regression fix.
> 

If I understand correctly the regression you talked about was introduced
by harsh punishment in XSA-39? If so, this is the patch you need to fix
that. Frontend only knows that it has connectivity or not. This patch
guarantee that the old netfront with larger MAX_SKB_FRAGS still see the
same thing from its point of view.  Netfront cannot know the
intermediate state between 18 and 20.

> As a separate fix we can consider increasing the number of slots
> per-packet once there is a mechanism to report this to the front end.
> 

Sure, that's on my TODO list.


Wei.

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