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Re: [Qemu-devel] Re: [PATCH 9/9] Introduce VLANClientState::cleanup()

From: Anthony Liguori
Subject: Re: [Qemu-devel] Re: [PATCH 9/9] Introduce VLANClientState::cleanup()
Date: Thu, 30 Apr 2009 11:32:51 -0500
User-agent: Thunderbird (X11/20090320)

Paul Brook wrote:
Sure, you can short-ciruit the case where you have a pair of devices,
but then you need to revoke posted receive descriptors.

Isn't the same true with a point-point API? When you add a third device to the vlan you're going to have to break the tap-guest link, and insert a proxy in the middle.

The difference is that if you assume vlan, then you have to handle renegotiation of features which is not actually possible. With virtio-net, once you enable guest GSO, you cannot disable it. You would have to implement GSO emulation within QEMU if you wanted to disable it on the tap device.

If you don't assume vlan, then you can choose the least amount of features when in a vlan. Then you never have to deal with this problem.

And yes, that means that if you wanted to support the existing syntax, you would need a flag like:

-net tap,i-will-never-add-more-devices-to-this-vlan=on -net nic,model=virtio

Although we should just add a new syntax. Honestly, vlans have to be the least useful feature in QEMU. Why in the world would anyone ever put more than one nic on a vlan? I understand why the bus architecture is appealing from a symmetry perspective but it's not at all useful from a user perspective.



Anthony Liguori

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