[dpdk-dev] [PATCH 2/2] vhost: start vhost servers once

Charles (Chas) Williams ciwillia at brocade.com
Fri Dec 30 22:26:27 CET 2016



On 12/29/2016 10:15 PM, Yuanhan Liu wrote:
> On Thu, Dec 29, 2016 at 10:58:11AM -0500, Charles (Chas) Williams wrote:
>> On 12/29/2016 03:52 AM, Yuanhan Liu wrote:
>>> On Wed, Dec 28, 2016 at 04:10:52PM -0500, Charles (Chas) Williams wrote:
>>>> Start a vhost server once during devinit instead of during device start
>>>> and stop.  Some vhost clients, QEMU, don't re-attaching to sockets when
>>>> the vhost server is stopped and later started.  Preserve existing behavior
>>>> for vhost clients.
>>>
>>> I didn't quite get the idea what you are going to fix.
>>
>> The issue I am trying to fix is QEMU interaction when DPDK's vhost is
>> acting as a server to QEMU vhost clients.  If you create a vhost server
>> device, it doesn't create the actual datagram socket until you call
>> .dev_start().  If you call .dev_stop() is also deletes those sockets.
>> For QEMU, this is a problem since QEMU doesn't know how to re-attach to
>> datagram sockets that have gone away.
>
> Thanks! And I'd appreciate it if you could have written the commit log
> this way firstly.
>
>> .dev_start()/.dev_stop() seems to roughly means link up and link down
>> so I understand why you might want to add/remove the datagram sockets.
>> However, in practice, this doesn't seem to make much sense for a DPDK
>> vhost server.
>
> Agree.
>
>> This doesn't seem like the right way to indicate link
>> status to vhost clients.
>>
>> It seems like it would just be easier to do this for both clients and
>> servers, but I don't know why it was done this way originally so I
>> choose to keep the client behavior.
>
> I don't think there are any differences between DPDK acting as client or
> server. To me, the right logic seems to be (for both DPDK as server and
> client).
>
> For register,
> - register the vhost socket at probe stage (either at rte_pmd_vhost_probe
>   or at eth_dev_vhost_create).
> - start the vhost session right after the register when we haven't started
>   it before.
>
> For unregister,
> - invoke rte_vhost_driver_unregister() at rte_pmd_vhost_remove().

OK. This will be much easier than what I submitted.

> For dev_start/stop,
> - set allow_queuing to 1/0 for start/stop, respectively.

Unfortunately, I don't think this will work.  new_device() doesn't happen
until a client connects.  allow_queueing seems to be following the status
of the "wire" as it where.  .dev_start()/.dev_stop() is the link of local
port connected to the wire (administratively up or down as it where).

.dev_start() can happen before new_device() and attempting to RX for a
client that doesn't exist doesn't seem like a good idea.  Perhaps another
flag that follows dev_started, but for the queues?


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