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Re: [Xen-devel] [PATCH net-next 0/2] xen-netback: Changes around carrier handling

On 06/08/14 00:07, David Miller wrote:
From: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
Date: Mon, 4 Aug 2014 16:20:56 +0100

This series starts using carrier off as a way to purge packets when the guest is
not able (or willing) to receive them. It is a much faster way to get rid of
packets waiting for an overwhelmed guest.
The first patch changes current netback code where it relies currently on
The second turns off the carrier if the guest times out on a queue, and only
turn it on again if that queue (or queues) resurrects.

Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>

Applied, but I have some reservations:

1) This is starting to bleed what is normally qdisc type policy into the

2) There are other drivers that could run into this kind of situation and
    have similar concerns, therefore we should make sure we have a consistent
    approach that such entities use to handle this problem.

Part of the problem is that netif_carrier_off() only partially mimicks
the situation.  It expresses the "transmitter is down so packets
aren't going onto the wire" part, which keeps the watchdog from
spitting out log messages ever time it fires.  But it doesn't deal
with packet freeing policy meanwhile, which I guess is the part that
this patch series is largely trying to address.

David Vrabel pointed out an important question in a reply to the previous version of this series: this patch deschedule NAPI if the carrier goes down. The backend doesn't receive packets from the guest. DavidVr and others said we shouldn't do this, the guest should be able to transmit even if it's not able/willing to receive. Other drivers doesn't deschedule NAPI at carrier off as well, however the "carrier off" information comes from the hardware, not from an untrusted guest who is not posting buffers on the receive ring. I don't have any good argument why I did it the current way, other than a hunch that it feels more natural.
David, do you have an opinion on that?


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