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

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

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().

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

	--yliu