netdev - Re: [net-next RFC V5 4/5] virtio

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Message-ID: <5016856D.8060009@gmail.com>
Date:	Mon, 30 Jul 2012 15:00:29 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Jason Wang <jasowang@...hat.com>, mashirle@...ibm.com,
	krkumar2@...ibm.com, habanero@...ux.vnet.ibm.com,
	rusty@...tcorp.com.au, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, edumazet@...gle.com,
	tahm@...ux.vnet.ibm.com, jwhan@...ewood.snu.ac.kr,
	davem@...emloft.net, akong@...hat.com, kvm@...r.kernel.org,
	sri@...ibm.com
Subject: Re: [net-next RFC V5 4/5] virtio_net: multiqueue support

On 07/29/2012 11:44 AM, Michael S. Tsirkin wrote:
> On Sat, Jul 21, 2012 at 02:02:58PM +0200, Sasha Levin wrote:
>> On 07/20/2012 03:40 PM, Michael S. Tsirkin wrote:
>>>> -	err = init_vqs(vi);
>>>>> +	if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ))
>>>>> +		vi->has_cvq = true;
>>>>> +
>>> How about we disable multiqueue if there's no cvq?
>>> Will make logic a bit simpler, won't it?
>>
>> multiqueues don't really depend on cvq. Does this added complexity really justifies adding an artificial limit?
> 
> Well !cvq support is a legacy feature: the reason we support it
> in driver is to avoid breaking on old hosts. Adding more code to that
> path just doesn't make much sense since old hosts won't have mq.

Is it really a legacy feature? The spec suggests that its an optional queue which is not necessary for the operation of the device.

Which is why we never implemented it in lkvm - we weren't interested in any of the features it provided at that time and we could provide high performance with vhost support even without it.
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