[dpdk-dev] [PATCH 2/3] net/virtio_user: fix wrong sequence of messages
Yuanhan Liu
yuanhan.liu at linux.intel.com
Fri Sep 9 06:19:49 CEST 2016
On Fri, Sep 09, 2016 at 11:59:18AM +0800, Tan, Jianfeng wrote:
> It's actually a good catch. After a light thought, I think in DPDK vhost, we
> may need to create those virtqueues once unix socket gets connected, just
> like in vhost-net, virtqueues are created on char file open. Right?
>
> There is a difference: for vhost-net and tap mode, IIRC, it knows how
> many queues before doing setup.
>
> No, from linux/drivers/vhost/net.c:vhost_net_open(), we can see that
> virtqueues are allocated according to VHOST_NET_VQ_MAX.
>
> Well, if you took a closer look, you will find VHOST_NET_VQ_MAX is
> defined to 2. That means it allocates a queue-pair only.
>
> And FYI, the MQ support for vhost-net is actually done in the tap
> driver, but not in vhost-net driver. That results to the MQ
> implementation is a bit different between vhost-net and vhost-user.
>
> Put simply, in vhost-net, one queue-pair has a backend fd associated
> with it. Vhost requests for different queue-pair are sent by different
> fd. That also means the vhost-net doesn't even need be aware of the
> MQ stuff.
>
> However, for vhost-user implementation, all queue-pairs share one
> socket fd. All requests all also sent over the single socket fd,
> thus the backend (DPDK vhost) has to figure out how many queue
> pairs are actually enabled: and we detect it by reading the
> vring index of SET_VRING_CALL message; it's not quite right though.
>
>
> Aha, right, nice analysis.
Just some rough memory in my mind ;-)
>
>
>
> How about reconsidering previous suggestion to allocate vq once connection
> is established?
>
> That's too much, because DPDK claims to support up to 0x8000
> queue-pairs. Don't even to say that the vhost_virtqueue struct
> was way too big before: it even holds an array of buf_vec with
> size 256.
>
>
> Another mistake of my memory, I was remember it wrongly as only 8 VQs are
> supported.
> One thing not related, provided that VHOST_MAX_QUEUE_PAIRS equals to 0x8000,
> struct vhost_virtqueue *virtqueue[VHOST_MAX_QUEUE_PAIRS * 2] spends 4MB for
> each virtio device, which could be a refined.
Yes, we could allocate a small array first, and then re-allocate if
necessary.
>
>
>
> Never mind, above fix on the vhost side will not take effect on existing
> vpp-vhost implementations.
>
> Actually, I was talking about the DPDK vhost implementation :)
>
>
> This patch is talking about vpp's native vhost implementation, not dpdk-vhost,
> and not the way vpp uses dpdk-vhost.
Yes, I know. What I meant is there was a "workaround" in DPDK vhost
implementation, and since you bring this issue on the table again,
it's a chance to think about how can we fix it.
A rough idea come to my mind is we could check all the per-vring message
at the begining of vhost_user_msg_handler() and allocate related vq when
necessary (when it's the first vring message we got).
Yeah, I know it's a bit ugly, but it at least gets rid of that "not-that-true"
assumption.
>
> Still not working. VPP needs SET_VRING_CALL to create vq firstly.
>
> Didn't get it. In the proposal, SET_FEATURES is sent before every other
> messages, thus it should not cause the issue you described in this patch.
>
>
> OK. Let me try to explain. We take three vhost implementations into
> consideration: dpdk-2.2-vhost, dpdk-master-vhost, vpp-native-vhost.
>
> If set_feature before set_vring_call, dpdk-2.2-vhost will fail: inside
> set_feature handler, assigning header length to VQs which will be created in
> set_vring_call handler.
Oh, right. That was an in-correct implementation.
> So we need to keep set_vring_call firstly.
> Then set_feature needs to be sent
> before any other msgs, this is what vpp-native-vhost requires. In all, the
> sequence is like this:
> 1. set_vring_call,
> 2. set_feature,
> 3. other msgs
>
>
> Besides, haven't we already sent SET_VRING_CALL before other messages
> (well, execept the SET_FEATURES and SET_MEM_TABLE message)?
>
>
> Yes, set_vring_call is already in the first place, but we need to plugin
> set_feature between set_vring_call and other msgs. Previously, set_vring_call
> and other msgs are together.
Okay. Another thing I noticed is that virtio-user lacks some feature
negotiations, like GET_FEATURES and GET_PROTOCOL_FEATURES. I think you
might need add them back somewhen?
--yliu
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