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

On Tue, 2013-11-19 at 16:42 +0000, Zoltan Kiss wrote:
> After further discussions and investigations, it seems it is a viable 
> approach to drop the packets in the RX path of the another VIF after a 
> timeout,

Since RX/TX in netback is a bit confusing (since it is inverted, but you
don't seem to be using it that way): A diagram:

domU (netfront) --> dom0 (netback) --> network stack --> bridge ,
domU' (netfront) <- timeout & drop <- dom0 (netback) <- stack <-'

You are proposing dropping at "timeout & drop". Since the dom0->domU'
path is based on copying there should be no problem with an skb getting
stuck with domU' holding on to it. In effect you will be dropping
traffic from some internal queue before it hits the shared ring anyway.
You will be making sure that either the full skb fits on the ring or it
remains in the queue.

What about any queueing which occurs in "network stack" (either
instance) or "bridge?" How can you cancel an skb out of those? Are you
intending that by dropping packets a "timeout & drop" they would
eventually make their way to the second netback and be droppable? How
convinced are you that this is viable?

>  and don't care about the rest of the cases (packets get stucked 
> somewhere in the core stack, a driver, or in the queue of a Dom0 
> userspace socket. In the latter case, they get copied anyway, so it 
> shouldn't happen)

I think that is OK iff you are copying for dom0 delivery. If you are not
copying here then an dom0 process (including an anonymous one) which can
open a socket and receive traffic could block things indefinitely.

The more general case of an unprivileged or deprivileged (i.e. a process
which has dropped its root privs somehow) being able to interfere with
the traffic in a way which causes gridlock might need a little more
thought though.

> Does anyone has a counterargument?
> Zoli
> On 13/11/13 20:29, Zoltan Kiss wrote:
> > 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?

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