Re: [PATCH 2/2] vhost: start vhost servers once

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Fri, Dec 30, 2016 at 04:26:27PM -0500, Charles (Chas) Williams wrote:
> 
> 
> 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.

Good.

> 
> >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. 

Right.

> Perhaps another
> flag that follows dev_started, but for the queues?

I will comment it on your v3 patches.

	--yliu