netdev - Re: [net-next rfc v7 2/3] virtio

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Message-ID: <50BC7896.4010504@redhat.com>
Date:	Mon, 03 Dec 2012 18:01:58 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Rusty Russell <rusty@...tcorp.com.au>, krkumar2@...ibm.com,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	bhutchings@...arflare.com, jwhan@...ewood.snu.ac.kr,
	shiyer@...hat.com
Subject: Re: [net-next rfc v7 2/3] virtio_net: multiqueue support

On 12/03/2012 05:47 PM, Michael S. Tsirkin wrote:
> On Mon, Dec 03, 2012 at 02:05:27PM +0800, Jason Wang wrote:
>> On Monday, December 03, 2012 12:34:08 PM Rusty Russell wrote:
>>> Jason Wang <jasowang@...hat.com> writes:
>>>> +static const struct ethtool_ops virtnet_ethtool_ops;
>>>> +
>>>> +/*
>>>> + * Converting between virtqueue no. and kernel tx/rx queue no.
>>>> + * 0:rx0 1:tx0 2:cvq 3:rx1 4:tx1 ... 2N+1:rxN 2N+2:txN
>>>> + */
>>>> +static int vq2txq(struct virtqueue *vq)
>>>> +{
>>>> +	int index = virtqueue_get_queue_index(vq);
>>>> +	return index == 1 ? 0 : (index - 2) / 2;
>>>> +}
>>>> +
>>>> +static int txq2vq(int txq)
>>>> +{
>>>> +	return txq ? 2 * txq + 2 : 1;
>>>> +}
>>>> +
>>>> +static int vq2rxq(struct virtqueue *vq)
>>>> +{
>>>> +	int index = virtqueue_get_queue_index(vq);
>>>> +	return index ? (index - 1) / 2 : 0;
>>>> +}
>>>> +
>>>> +static int rxq2vq(int rxq)
>>>> +{
>>>> +	return rxq ? 2 * rxq + 1 : 0;
>>>> +}
>>>> +
>>> I thought MST changed the proposed spec to make the control queue always
>>> the last one, so this logic becomes trivial.
>> But it may break the support of legacy guest. If we boot a legacy single queue 
>> guest on a 2 queue virtio-net device. It may think vq 2 is cvq which is indeed 
>> rx1.
> Legacy guyest support should be handled by host using feature
> bits in the usual way: host should detect legacy guest
> by checking the VIRTIO_NET_F_RFS feature.
>
> If VIRTIO_NET_F_RFS is acked, cvq is vq max_virtqueue_pairs * 2.
> If it's not acked, cvq is vq 2.
>

We could, but we didn't gain much from this. Furthermore, we need also do the dynamic creation/destroying of virtqueues during feature negotiation which seems not supported in qemu now.
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