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Date: Mon, 12 Apr 2010 22:49:47 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] How to lock-up your tap-based VM network
Message-ID: <20100412214947.GC6148@shareable.org>
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	<201004122107.19425.paul@codesourcery.com>
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To: Paul Brook <paul@codesourcery.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>, qemu-devel@nongnu.org

Paul Brook wrote:
> > A major reason for this deadlock could likely be removed by shutting
> > down the tap (if peered) or dropping packets in user space (in case of
> > vlan) when a NIC is stopped or otherwise shut down. Currently most (if
> > not all) NIC models seem to signal both "queue full" and "RX disabled"
> > via !can_receive().
> 
> No. A disabled device should return true from can_recieve, then discard the 
> packets in its receive callback. Failure to do so is a bug in the device. It 
> looks like the virtio-net device may be buggy.

I agree - or alternatively signal that there's no point sending it
packets and they should be dropped without bothering to construct them.

But anyway, this flow control mechanism is buggy - what if instead of
an interface down, you just have a *slow* guest?  That should not push
back so much that it makes other guests networking with each other
slow down.

-- Jamie