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Re: [Xen-devel] netback: Delayed copy alternative

> -----Original Message-----
> From: Zoltan Kiss
> Sent: 13 November 2013 20:30
> To: xen-devel@xxxxxxxxxxxxxxxxxxxx; Ian Campbell; Wei Liu; Paul Durrant;
> Malcolm Crossley; David Vrabel
> Cc: Jonathan Davies
> Subject: netback: Delayed copy alternative
> 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?

Well, now that David fixed the DMA unmap tracking thing, I believe that another 
vif is *generally* the only place an skb can hang around for a long time. The 
problem is that there is an edge case... If a network driver turns off queue 
processing (for flow control reasons, and NB that 10G Ethernet requires the 
driver to do this if the PHY signals flow control and internal buffering is 
exhausted, 1G is allowed to be an open drain) then the skb can sit in the queue 
indefinitely and there's no way you can deal with this from the guest RX side 
of netback. You need to have a copy-aside option to handle this.


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