netback: Delayed copy alternative

[Zoltan Kiss <zoltan.kiss@citrix.com>]

Hi,

I'm trying to forward port delayed copy to my new grant mapping patches. 
One important problem I've faced is that classic used 
gnttab_copy_grant_page to replace the granted page with a local copy and 
unmap the grant. And this function has never been upstreamed as only 
netback used it. Unfortunately upstreaming it is not a very easy task, 
as the kernel's grant table infrastructure doesn't track at the moment 
whether the page is DMA mapped or not. It is required because we 
shouldn't proceed with the copy and replace if a device already mapped 
the page for DMA.
David came up with an alternative idea: we do this delayed copy because 
we don't want the guest's page to get stucked in Dom0 indefinitely. The 
only realistic case for that would be if the egress interface would be 
an another guest's vif, where the guest (either due to a bug or as a 
malicious attempt) doesn't empty its ring. I think it's a safe 
assumption that Dom0 otherwise doesn't hold on to packets for too long. 
Or if it does, then that's a bug we should fix instead of doing a copy 
of the packet.
If we accept that only other vif's can keep the skb indefinitely, then 
an easier solution would be to handle this problem on the RX side: the 
RX thread can also check whether this skb hanged around for too long and 
drop it. Actually, xenvif_start_xmit already checks if the guest 
provided enough slots for us to do the grant copy. If I understand it 
correctly. What do you think about such an approach?

Regards,

Zoli