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From: Markus Armbruster <armbru@redhat.com>
Subject: Re: [Qemu-devel] [PATCH v2] net: Fix hotplug with pci_add
References: <356ef9bdde008d695e7c75dd1566222d6160d4b6.1276011638.git.amit.shah@redhat.com>
	<m3zkz55vsz.fsf@blackfin.pond.sub.org>
	<20100608172525.GB10305@amit-laptop.redhat.com>
Date: Wed, 09 Jun 2010 08:37:26 +0200
In-Reply-To: <20100608172525.GB10305@amit-laptop.redhat.com> (Amit Shah's
	message of "Tue, 8 Jun 2010 22:56:11 +0530")
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To: Amit Shah <amit.shah@redhat.com>
Cc: Juan Quintela <quintela@redhat.com>, Gerd Hoffmann <kraxel@redhat.com>, qemu list <qemu-devel@nongnu.org>

Amit Shah <amit.shah@redhat.com> writes:

> On (Tue) Jun 08 2010 [18:33:00], Markus Armbruster wrote:
>> Amit Shah <amit.shah@redhat.com> writes:
>> 
>> > The correct model type wasn't getting added when hotplugging nics with
>> > pci_add.
>> >
>> > Testcase: start VM with default nic type. In the qemu_monitor:
>> >
>> > (qemu) pci_add auto nic model=virtio
>> >
>> > This results in a nic hot-plug of the same nic type as the default.
>> 
>> Works fine for me on master, fd1dc858370d9a9ac7ea2512812c3a152ee6484b.
>> What am I doing wrong?
>
> Did you start with a virtio nic added? The 'default' here is the nic
> type that's added as the first nic. Try this: start a VM with model
> e1000 and use pci_add to add a nic type of virtio.

I do.  Nevertheless, "pci_add auto nic model=e1000" adds an e1000.

Also works if I start without a NIC.

Ah, if I start with a -net nic, then pci_add breaks as described!

Now my next question is *how* your patch fixes this.  It's not at all
obvious to me.

>> > This was broken in 5294e2c774f120e10b44652ac143abda356f44eb
>> >
>> > Also changes the behaviour where no .init is defined for a
>> > net_client_type. Previously, 0 was returned, which indicated the init
>> > was successful and that 0 was the index into the nd_tables[] array.
>> > Return -1, indicating unsuccessful init, in such a case.
>> 
>> The only element of net_client_types[] without an init() method is type
>> "none", index 0.  So, doesn't this break -net none?  And what does it
>> fix?
>
> The net_client_types[] index isn't relevant here. -net none works fine,
> no problem.

Let me rephrase: Behavior changes for -net types without an init()
method.  The only one without an init() method is "none".  Before,
net_client_init() succeeded for it.  Now it fails.  What's the impact of
that change?  And why does it make sense?