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Re: [Qemu-devel] How to lock-up your tap-based VM network


From: Jan Kiszka
Subject: Re: [Qemu-devel] How to lock-up your tap-based VM network
Date: Tue, 13 Apr 2010 14:19:53 +0200
User-agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv:1.8.1.12) Gecko/20080226 SUSE/2.0.0.12-1.1 Thunderbird/2.0.0.12 Mnenhy/0.7.5.666

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.

That's not a virtio-only issue. In fact, we ran into this over pcnet,
and a quick check of other popular PCI NIC models (except for rtl8139)
revealed the same picture: They only report can_receive if their
receiver unit is up and ready (some also include the queue state, but
that's an "add-on").

I think it's clear why: "can_receive" strongly suggests that a suspended
receiver should make the model return false. If we want to keep this
handler, it should be refactored to something like "queue_full".

But before starting any refactoring endeavor: Do we have a consensus on
the direction? Refactor can_receive to queue_full? Or even drop it?

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux




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